Check correct directory deletion behaviour for simple project

-  The integration test ensures the correct directories are present,
then deleted, for a project with only one package, not in a
subdirectory.

test/integration/tests/3863-purge-command/Main.hs

@@ -1,7 +1,39 @@
 import StackTest
+import Data.Maybe (listToMaybe, fromMaybe)
 import System.Directory
+import System.FilePath
 
 main :: IO ()
-main = do
-  stack ["build"]
-  stack ["purge"]
+main =
+  -- For these commands, we'll need to know the `dist` directory.
+  -- This is usually `.stack-work/dist/$compiler-variant/Cabal-xxxx`
+  stackCheckStdout [defaultResolverArg, "path", "--dist-dir"] $ \distDir ->
+
+    stackCheckStdout [defaultResolverArg, "path", "--local-install-root"] $ \localInstallRoot -> do
+
+      -- Usually `.stack-work`
+      let stackWork = fromMaybe (error "There must be a stack working directory.") $
+            listToMaybe (splitDirectories distDir)
+
+      -- First, clean the .stack-work directory.
+      -- This is only necessary when running individual tests.
+      stack [defaultResolverArg, "purge"]
+      doesNotExist stackWork
+
+      -- The dist directory should exist after a build
+      stack [defaultResolverArg, "build"]
+      doesExist distDir
+      doesExist localInstallRoot
+      doesExist stackWork
+
+      -- The dist directory should not exist after a clean, whereas the
+      -- .stack-work directory should
+      stack [defaultResolverArg, "clean"]
+      run "exa" ["-T", ".stack-work"]
+      doesNotExist distDir
+      doesExist localInstallRoot
+      doesExist stackWork
+
+      -- The .stack-work directory should not exist after a purge
+      stack [defaultResolverArg, "purge"]
+      doesNotExist stackWork