Restore clean Thread class after applying Profiling thread extensions

[delner]

After adding a type check in #1200, it became clear that profiling tests would leave the Thread in an unclean state. Specifically, Thread would be replaced with a duplicate of the original class. Although this was functionally similar to the original Thread class object, the object_id is different, and this was causing Thread.current.class == Thread to evaluate as false.

In this pull request, we want to leave the Thread constant in a pristine state after a profiling test is complete, so that subsequent tests are consistent and dependable. To do this, we leverage stub_const to temporarily replace the Thread constant with a copy of Thread and restore it after the test.

There's also a specific case where we must profiling on the main Thread object. This main thread object is an instance of the original Thread class, not our stubbed copy of the Thread class, which means in our test suite, we cannot update this object without altering the original Thread class (something we cannot do if we want to preserve pristine state.) Instead in these tests, we must fork then patch the main thread inside the fork, to allow profiling on the main thread to be tested without altering the original Thread class.