Fix failing scheduler unit test

[Kami]

Some of the scheduler unit tests (tests.unit.test_scheduler file) occasionally randomly fail which causes the Travis build failure.

I didn't have the time do dig in and find the root cause - aka if it's a test issue or there is also an underlying race / issue in the code.

Here is the output - https://gist.github.com/Kami/00397cb3b100cbbba9ba4313d8f24fb3