Rename the changelog with *.md extension

- Add a section on cabal-testsuite changes

changelog.d/pr-10646.md

@@ -0,0 +1,71 @@
+---
+synopsis: Configuration messages without duplicates
+packages: [cabal-install-solver]
+prs: 10646
+issues: 10645
+---
+
+The "using configuration from" message no longer has duplicates on Windows when
+the `cabal.project` uses forward slashes for its imports but the message reports
+imports with backslashes.
+
+```diff
+$ cat cabal.project
+import: dir-a/b.config
+
+$ cabal build all --dry-run
+...
+When using configuration from:
+-   - dir-a/b.config
+    - dir-a\b.config
+    - cabal.project
+```
+
+## Changed `Ord ProjectConfigPath` Instance
+
+For comparison purposes, path separators are normalized to the `buildOS`
+platform's path separator.
+
+```haskell
+-- >>> let abFwd = ProjectConfigPath $ "a/b.config" :| []
+-- >>> let abBwd = ProjectConfigPath $ "a\\b.config" :| []
+-- >>> compare abFwd abBwd
+-- EQ
+```
+
+## Changes in `cabal-testsuite`
+
+With `ghc-9.12.1` adding `-XMultilineStrings` it would be easier to write
+multiline strings in `cabal-testsuite/PackageTests/**/*.test.hs` scripts but the
+catch is we run these tests with older `GHC` versions so would need to use
+`-XCPP` for those versions and the C preprocessor does not play nicely with
+string gaps. To avoid these problems a `readVerbatimFile` function can be used
+to read the expected multiline output for tests verbatim. This has the same
+implementation as `readFile` from the `strict-io` package to avoid problems at
+cleanup.
+
+```
+Warning: Windows file locking hack: hit the retry limit 3 while trying to
+remove C:\Users\phild\AppData\Local\Temp\cabal-testsuite-8376
+cabal.test.hs:
+C:\Users\<username>\AppData\Local\Temp\cabal-testsuite-8376\errors.expect.txt:
+removePathForcibly:DeleteFile
+"\\\\?\\C:\\Users\\<username>\\AppData\\Local\\Temp\\cabal-testsuite-8376\\errors.expect.txt":
+permission denied (The process cannot access the file because it is being
+used by another process.)
+```
+
+The other process accessing the file is `C:\WINDOWS\System32\svchost.exe`
+running a `QueryDirectory` event and this problem only occurs when the test
+fails.
+
+The `assertOutputContains` function was modifying the output but in a way not
+"visible" enough. An added `assertOn` function (that `assertOutputContains`
+calls) takes a `NeedleHaystack` configuration for how the search is made, what
+to expect and and how to display the expected and actual values. A pilcrow ¶ is
+often used to visibly display line endings but our terminal output is restricted
+to ASCII so lines are visibly delimited between `^` and `$` visible markers. The
+needle (the expected output fragment) is shown annotated this way and the
+haystack (the output) can optionally be shown this way too.
+
+The `concatOutput` function is renamed to `lineBreaksToSpaces`.