More consistently pass --package-db flag to tests (backport #9954)

[mergify]

Previously --package-db was only passed to test which used the v2- prefix.

Now we pass --package-db to things which use the v2- prefix, not things which use the v1- prefix and by default assume that unprefixed commands are v2 commands.

Please read Github PR Conventions and then fill in one of these two templates.

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

---

This is an automatic backport of pull request #9954 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2b44677 has failed:

On branch mergify/bp/3.12/pr-9954
Your branch is up to date with 'origin/3.12'.

You are currently cherry-picking commit 2b44677c3.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   cabal-testsuite/PackageTests/Regression/T9640/cabal.out
	modified:   cabal-testsuite/src/Test/Cabal/DecodeShowBuildInfo.hs
	modified:   cabal-testsuite/src/Test/Cabal/Prelude.hs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cabal-testsuite/PackageTests/CustomTestCoverage/cabal.out

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally