testsuite: withShorterPathForNewBuildStore and findDependencyInStore should be more abstract wrt store directory

`withShorterPathForNewBuildStore` and `findDependencyInStore` should live in the `TestM` monad and the author of the test should be unaware of how the `--store-dir` argument is passed to cabal.

This would remove the need to explicitly pass the `--store-dir` option in tests which need `withShorterPathForNewBuildStore` and also remove the need to explicitly pass the store dir argument to findDependencyInStore. Not passing these arguments is a potential source of bugs.

I have a patch, which I will put up after #9454 is merged.