fix: properly handle XTestGoFiles (#225)

lua/neotest-golang/runspec/file.lua

@@ -50,6 +50,16 @@ function M.build(pos, tree, strategy)
         break
       end
     end
+    if golist_item.XTestGoFiles ~= nil then
+      -- NOTE: XTestGoFiles are test files that are part of a [packagename]_test package.
+      if
+        pos_path_foldername == golist_item.Dir
+        and vim.tbl_contains(golist_item.XTestGoFiles, pos_path_filename)
+      then
+        package_name = golist_item.ImportPath
+        break
+      end
+    end
   end
 
   -- find all top-level tests in pos.path

tests/go/testify/lookup_spec.lua → tests/go/lookup_spec.lua

@@ -36,6 +36,14 @@ describe("Lookup", function()
         package = "main",
         replacements = {},
       },
+      [folderpath .. "/x/xtest_blackbox_test.go"] = {
+        package = "x_test",
+        replacements = {},
+      },
+      [folderpath .. "/x/xtest_whitebox_test.go"] = {
+        package = "x",
+        replacements = {},
+      },
     }
 
     -- Act

tests/go/x/xtest.go

@@ -0,0 +1,4 @@
+package x
+
+func Add(a, b int) int { return a + b }
+func internal() string { return "private" }

tests/go/x/xtest_blackbox_test.go

@@ -0,0 +1,14 @@
+package x_test
+
+import (
+	"testing"
+
+	"github.com/fredrikaverpil/neotest-golang/x"
+)
+
+func TestBlackBox(t *testing.T) {
+	// Can only access Add() through the public interface of x.
+	if x.Add(1, 2) != 3 {
+		t.Fail()
+	}
+}

tests/go/x/xtest_whitebox_test.go

@@ -0,0 +1,13 @@
+package x
+
+import "testing"
+
+func TestWhiteBox(t *testing.T) {
+	// Can access both Add() and internal()
+	if Add(1, 2) != 3 {
+		t.Fail()
+	}
+	if internal() != "private" {
+		t.Fail()
+	}
+}