Refactor compile-related tests to share construction. (#4396)

Note in particular that this fixes an issue where SharedValueStore had
been shared across files, when they should be per-file. This is only
visible when doing multiple compilations in a single test, which was
rare before.

This also moves these tests into the Testing namespace. My memory of the
various namespacing changes is that we'd generally agreed to have tests
in Testing so that we'd see SemIR:: and similar, same as we would in a
lot of the implementation.

toolchain/lex/BUILD

@@ -257,6 +257,7 @@ cc_test(
         "//toolchain/base:value_store",
         "//toolchain/diagnostics:diagnostic_emitter",
         "//toolchain/diagnostics:mocks",
+        "//toolchain/testing:compile_helper",
         "//toolchain/testing:yaml_test_helpers",
         "@googletest//:gtest",
         "@llvm-project//llvm:Support",

toolchain/parse/BUILD

@@ -45,6 +45,7 @@ cc_test(
         "//toolchain/diagnostics:mocks",
         "//toolchain/lex",
         "//toolchain/lex:tokenized_buffer",
+        "//toolchain/testing:compile_helper",
         "@googletest//:gtest",
     ],
 )
@@ -139,6 +140,7 @@ cc_test(
         "//toolchain/diagnostics:mocks",
         "//toolchain/lex",
         "//toolchain/lex:tokenized_buffer",
+        "//toolchain/testing:compile_helper",
         "//toolchain/testing:yaml_test_helpers",
         "@googletest//:gtest",
         "@llvm-project//llvm:Support",

toolchain/parse/tree_test.cpp

@@ -17,6 +17,7 @@
 #include "toolchain/lex/tokenized_buffer.h"
 #include "toolchain/parse/parse.h"
 #include "toolchain/parse/tree_and_subtrees.h"
+#include "toolchain/testing/compile_helper.h"
 #include "toolchain/testing/yaml_test_helpers.h"
 
 namespace Carbon::Parse {
@@ -30,38 +31,19 @@ namespace Yaml = ::Carbon::Testing::Yaml;
 
 class TreeTest : public ::testing::Test {
  protected:
-  auto GetSourceBuffer(llvm::StringRef t) -> SourceBuffer& {
-    CARBON_CHECK(fs_.addFile("test.carbon", /*ModificationTime=*/0,
-                             llvm::MemoryBuffer::getMemBuffer(t)));
-    source_storage_.push_front(
-        std::move(*SourceBuffer::MakeFromFile(fs_, "test.carbon", consumer_)));
-    return source_storage_.front();
-  }
-
-  auto GetTokenizedBuffer(llvm::StringRef t) -> Lex::TokenizedBuffer& {
-    token_storage_.push_front(
-        Lex::Lex(value_stores_, GetSourceBuffer(t), consumer_));
-    return token_storage_.front();
-  }
-
-  SharedValueStores value_stores_;
-  llvm::vfs::InMemoryFileSystem fs_;
-  std::forward_list<SourceBuffer> source_storage_;
-  std::forward_list<Lex::TokenizedBuffer> token_storage_;
-  DiagnosticConsumer& consumer_ = ConsoleDiagnosticConsumer();
+  Testing::CompileHelper compile_helper_;
 };
 
 TEST_F(TreeTest, IsValid) {
-  Lex::TokenizedBuffer& tokens = GetTokenizedBuffer("");
-  Tree tree = Parse(tokens, consumer_, /*vlog_stream=*/nullptr);
+  Tree& tree = compile_helper_.GetTree("");
   EXPECT_TRUE((*tree.postorder().begin()).is_valid());
 }
 
 TEST_F(TreeTest, AsAndTryAs) {
-  Lex::TokenizedBuffer& tokens = GetTokenizedBuffer("fn F();");
-  Tree tree = Parse(tokens, consumer_, /*vlog_stream=*/nullptr);
+  auto [tokens, tree_and_subtrees] =
+      compile_helper_.GetTokenizedBufferWithTreeAndSubtrees("fn F();");
+  const auto& tree = tree_and_subtrees.tree();
   ASSERT_FALSE(tree.has_errors());
-  TreeAndSubtrees tree_and_subtrees(tokens, tree);
   auto it = tree_and_subtrees.roots().begin();
   // A FileEnd node, so won't match.
   NodeId n = *it;
@@ -105,11 +87,11 @@ TEST_F(TreeTest, AsAndTryAs) {
 }
 
 TEST_F(TreeTest, PrintPostorderAsYAML) {
-  Lex::TokenizedBuffer& tokens = GetTokenizedBuffer("fn F();");
-  Tree tree = Parse(tokens, consumer_, /*vlog_stream=*/nullptr);
-  EXPECT_FALSE(tree.has_errors());
+  auto [tokens, tree_and_subtrees] =
+      compile_helper_.GetTokenizedBufferWithTreeAndSubtrees("fn F();");
+  EXPECT_FALSE(tree_and_subtrees.tree().has_errors());
   TestRawOstream print_stream;
-  tree.Print(print_stream);
+  tree_and_subtrees.tree().Print(print_stream);
 
   auto file = Yaml::Sequence(ElementsAre(
       Yaml::Mapping(ElementsAre(Pair("kind", "FileStart"), Pair("text", ""))),
@@ -126,17 +108,17 @@ TEST_F(TreeTest, PrintPostorderAsYAML) {
       Yaml::Mapping(ElementsAre(Pair("kind", "FileEnd"), Pair("text", "")))));
 
   auto root = Yaml::Sequence(ElementsAre(Yaml::Mapping(
-      ElementsAre(Pair("filename", "test.carbon"), Pair("parse_tree", file)))));
+      ElementsAre(Pair("filename", tokens.source().filename().str()),
+                  Pair("parse_tree", file)))));
 
   EXPECT_THAT(Yaml::Value::FromText(print_stream.TakeStr()),
               IsYaml(ElementsAre(root)));
 }
 
 TEST_F(TreeTest, PrintPreorderAsYAML) {
-  Lex::TokenizedBuffer& tokens = GetTokenizedBuffer("fn F();");
-  Tree tree = Parse(tokens, consumer_, /*vlog_stream=*/nullptr);
-  EXPECT_FALSE(tree.has_errors());
-  TreeAndSubtrees tree_and_subtrees(tokens, tree);
+  auto [tokens, tree_and_subtrees] =
+      compile_helper_.GetTokenizedBufferWithTreeAndSubtrees("fn F();");
+  EXPECT_FALSE(tree_and_subtrees.tree().has_errors());
   TestRawOstream print_stream;
   tree_and_subtrees.PrintPreorder(print_stream);
 
@@ -167,7 +149,8 @@ TEST_F(TreeTest, PrintPreorderAsYAML) {
                                 Pair("kind", "FileEnd"), Pair("text", "")))));
 
   auto root = Yaml::Sequence(ElementsAre(Yaml::Mapping(
-      ElementsAre(Pair("filename", "test.carbon"), Pair("parse_tree", file)))));
+      ElementsAre(Pair("filename", tokens.source().filename().str()),
+                  Pair("parse_tree", file)))));
 
   EXPECT_THAT(Yaml::Value::FromText(print_stream.TakeStr()),
               IsYaml(ElementsAre(root)));
@@ -178,7 +161,7 @@ TEST_F(TreeTest, HighRecursion) {
   code.append(10000, '(');
   code.append(10000, ')');
   code += "; }";
-  Lex::TokenizedBuffer& tokens = GetTokenizedBuffer(code);
+  Lex::TokenizedBuffer& tokens = compile_helper_.GetTokenizedBuffer(code);
   ASSERT_FALSE(tokens.has_errors());
   Testing::MockDiagnosticConsumer consumer;
   Tree tree = Parse(tokens, consumer, /*vlog_stream=*/nullptr);