[lldb][test] Add extra logging for module cache test

And remove the workaround I was trying, as this logging may prove what
the actual issue is.

Which I think is that the thread plan map in Process is cleared before
the threads are destroyed. So Thread::ShouldStop could be getting
the current plan, then the plan map is cleared, then Thread::ShouldStop
is deciding based on that plan to pop a plan from the now empty stack.