Make tests run in parallel
Currently our test suite runs in one core. By using parallel tests we can make it run in as many process as cores in the machine, which should yield much faster running times.
Fixes #10106 - Make tests run in parallel
Run rake parallel:test to run tests in several processes. The gem will
auto split the tests in as many cores as you have available.
Keep in mind running tests in parallel could highlight race conditions
we would've missed otherwise, so some tests might to be fixed to ensure
any process can pick it up and run it.