[lldb] Add assertStopReason helper function

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 ce825e46.

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