testsuite: Refactor withShorterPathForNewBuildStore (backport #9505) (#10059)

* testsuite: Refactor withShorterPathForNewBuildStore

This makes `withShorterPathForNewBuildStore` fit more nicely into the
rest of the testing infrastructure.

* Move `withShorterPathForNewBuildStore` to `TestM` monad
* Move responsibility for passing `--store-dir` to `cabalGArgs` function
* Move `findDependencyInStore` into `TestM`, and remove requirement to
  pass path to store directory.
* Introduce `testStoreDir` function which returns the store location
  (and honours `withShorterPathForNewBuildStore`)
* Migrate tests which use `withShorterPathForNewBuildStore`.

(cherry picked from commit 2a2d0b30)

# Conflicts:
#	Cabal-tests/Cabal-tests.cabal
#	Cabal-tests/lib/Test/Utils/TempTestDir.hs

* !fixup resolve conflicts

* fixup! always import `(</>)`

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Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>
Co-authored-by: Artem Pelenitsyn <a.pelenitsyn@gmail.com>
Co-authored-by: brandon s allbery kf8nh <allbery.b@gmail.com>