getcwd fixes

[jtwaleson]

While running tests I noticed some failures.

sh: 0: getcwd() failed: No such file or directory

and

failures:
    bgworker::pg::tests::can_cancel_job_by_name
    bgworker::pg::tests::can_clear
    bgworker::pg::tests::can_clear_by_status
    bgworker::pg::tests::can_complete_job_with_interval
    bgworker::pg::tests::can_complete_job_without_interval
    bgworker::pg::tests::can_fail_job
    bgworker::pg::tests::can_get_jobs

Narrowed this down to two points:

• the cargo_config tests change the CWD of the process and don't restore it
• the postgres init code relies on CWD

This changes solves both (although one would be enough): we restore CWD and make the fixture init independent of the current working dir.

We could also run the cargo_config tests in serial mode with serial_test but I opted to not use that as it would be an extra dependency and we're not using that.