Add a function to make it easier to debug a test failure caused by an
unexpected stop reason. This is similar to the assertState helper that
was added in ce825e4674.
Before:
self.assertEqual(stop_reason, lldb.eStopReasonInstrumentation)
AssertionError: 5 != 10
After:
self.assertStopReason(stop_reason, lldb.eStopReasonInstrumentation)
AssertionError: signal (5) != instrumentation (10)
Differential revision: https://reviews.llvm.org/D131083