Allows for using `load` with an absolute path instead of a relative one, which makes it possible to group tests into different directories.
Parallel tests differ slightly, loading the newer `helper/common.bash` and `helper/setup.bash` files instead of the older `test_helper/common.bash` which serial tests continue to use.
- `test_helper.bats` needs more work than this PR provides to be compatible with parallel tests, so must remain as a serial test for now.
- `spam_bounced.bats` had failures as a serial test, but works well converted to a parallel test in a future commit.
