Backport #10646: Deduplicate path separator duplicates

[mergify]

Fixes #10645, a small Windows gotcha missed with #10546.

I really had to get into the weeds with this one. The assertOutputContains function was modifying the output (a test actual value) before comparision with the unmodified actual value. This change was not visible. I've added an assertOn function (that assertOutputContains calls) that takes a NeedleHaystack configuration for how the search, expectation and display are made. I had wanted to add a pilcrow ¶ for line endings but the terminal output is restricted to ASCII so I've marked line beginnings with ^ and line endings with $. The needle (the expected output fragment) is shown annotated this way as can the haystack (the output). The haystack is not shown annotated with line delimiters but can be.

The concatOutput function, I've renamed to lineBreaksToSpaces. There's more detail on the cabal-testsuite changes in the changelog entry. One benefit of using an .md extension for changelog.d/pr-10646.md is that typos are caught by CI.

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. Yes, a doctest.

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This is an automatic backport of pull request #10646 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d0bd687 has failed:

On branch mergify/bp/3.14/pr-10646
Your branch is up to date with 'origin/3.14'.

You are currently cherry-picking commit d0bd687a2.
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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs

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Cherry-pick of 30f5d3f has failed:

On branch mergify/bp/3.14/pr-10646
Your branch is ahead of 'origin/3.14' by 1 commit.
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Changes to be committed:
	new file:   cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.out
	new file:   cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.test.hs
	new file:   cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/dir-else/else.config
	new file:   cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/else.project
	new file:   cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/msg.expect.txt
	modified:   cabal-testsuite/README.md
	new file:   cabal-testsuite/src/Test/Cabal/NeedleHaystack.hs
	new file:   changelog.d/pr-10646.md

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CONTRIBUTING.md
	both modified:   cabal-testsuite/cabal-testsuite.cabal
	both modified:   cabal-testsuite/src/Test/Cabal/Prelude.hs

Cherry-pick of 484ac9c has failed:

On branch mergify/bp/3.14/pr-10646
Your branch is ahead of 'origin/3.14' by 2 commits.
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You are currently cherry-picking commit 484ac9cb0.
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Changes to be committed:
	modified:   cabal-testsuite/PackageTests/BuildDeps/InternalLibrary2/setup.test.hs
	modified:   cabal-testsuite/PackageTests/BuildDeps/InternalLibrary3/setup.test.hs
	modified:   cabal-testsuite/PackageTests/CheckSetup/setup.test.hs
	modified:   cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs
	modified:   cabal-testsuite/PackageTests/NewBuild/T4288/cabal.test.hs
	modified:   cabal-testsuite/PackageTests/Regression/T5409/use-different-versions-of-dependency-for-library-and-build-tool.test.hs

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromComplex/cabal.out
	deleted by us:   cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromComplex/cabal.test.hs

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