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.

Change-Id: Ic20826af57c650a79580e907986eb3b0204a0a18
Signed-off-by: Jeff Epler <623a871f32baaca066adfe0cac08d7aa7d34027d@gmail.com>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython