Change directory more safely in unit tests

[Iain-S]

A slight improvement over the previous system since we now save the starting directory in setUp and return to it in tearDown. Previously, we did chdir('../..') in tearDown, which could go wrong if something unexpected happened in setUp or if an extra chdir() was used in the test functions themselves.

Some improvements that could be added here or in a future PR

• Reduce duplication by moving the examples/ and workspace/ directories to the test utils module and making them Final.
• run_psql() presumes that it is being called from the repo root directory. This doesn't feel good as it then matters which order you do the setUp calls in. Is there a way we can remove this reliance on the current working directory to run the db setup scripts with psql?

[Iain-S]

Update:

• I have added a commit that changes how run_psql() works slightly. It can now be called from any location and takes a Path to the .dmp file to be run.