Fix some intermittent test failures

Some tests leak Skylight::Instrumenter.current_trace, this can cause
future tests to fail. This commit attempts to clean up these cases, but
the problem should be revisited.

Since trace builders hold a reference to the instrumenter, it should be
possible to check whether or not the instrumenter has been dropped and
restarted and lazily clear stale trace builder objects.
Make double sure that Trace started_at is an Integer