Add Thread tests

[tmickel]

Currently, there are no tests for Thread. There are a bunch of methods in there that could be easily unit-tested and are pretty critical for managing execution.

[SillyInventor]

I'm interested in adding some thread testing, would you like a hand?

[thisandagain]

@SillyInventor Yeah! That would be great.

[thisandagain]

We are still not at 100% coverage for engine/thread but #445 gets us close enough to close this for now.