tests/run-tests.py: Parallelize running tests by default.
tests/run-tests.py: Parallelize running tests by default.

This significantly reduces the time taken to run the test suite (on the
unix port).  Use `-j1` to disable this feature.

Signed-off-by: Jeff Epler <623a871f32baaca066adfe0cac08d7aa7d34027d@gmail.com>

adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython