Fix a problem with "watchpoint triggers before" watchpoint handling.

We need to step the watchpoint instruction in these cases, but the
when we queued the ThreadPlanStepOverWatchpoint to do this, we didn't
make it a Controlling plan.  So if you are stepping, this plan returns as
though it were a utility plan, and the stepping plan keeps going.

This only partially fixes the problem on Darwin; there's another bug
with reporting a watchpoint when we're instruction single stepping over
an instruction that triggers a watchpoint.  The kernel reports the
"single step completed" but not the watchpoint hit.  So this commit
also refactors the test into a part that works (at least on Darwin) and
a part that still fails.

We may have to adjust the test result expectations for other systems after
this fix.

Differential Revision: https://reviews.llvm.org/D146337