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`.