Tests: Explicitly set multiprocessing start method to "fork"
Tests: Explicitly set multiprocessing start method to "fork"

By using multiprocessing's ForkProcess, the testsuite will also start
working on macOS again, even on Python 3.8+.

The background on this is that beginning with Python 3.8+, "fork" is not
the default on all platforms anymore but has been changed to "spawn" on
macOS. This method, however, apparently is not compatible with how 
multiprocessing is currently used within the test harness.
crate/crate-python,crate/crate-python