Reset current directory after test completion

[islas]

When running multiple tests with mapped relative directories, the working directory will be changed to the mapped directory. However, if running in a process pool, the next test to pick up a worker that has already run with a different working directory will start in that wrong directory and thus the relative directory change will not be executed from the expected location.

To ensure processes in a pool always hand off at a known directory the current directory is stashed and subsequently popped after testing, regardless of test outcome.