[philderbeast]

Fixes #12506 and avoids the manual synchronisation task noted in this code snippet:

cabal/cabal-testsuite/src/Test/Cabal/Monad.hs

Lines 596 to 602 in 1795db0

	-- NB: Keep this synchronized with partitionTests
	isTestFile :: FilePath -> Bool
	isTestFile f =
	case takeExtensions f of
	".test.hs" -> True
	".multitest.hs" -> True
	_ -> False

Many of the newly found tests are marked with expectBroken 12501.

I'll squash commits before applying the merge label if this pull request is approved.

---

• 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. (Yes - in cabal-testsuite/README.md)
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)