de-flake TestThreadStates.test_process_interrupt

the "self.assertEqual(thread.GetStopReason(), lldb.eStopReasonSignal)"
was occasionally failing because the stop reason would come out as
"trace" this happened if we issued the interrupt just as the processed
stopped due to single-stepping over the breakpoint (i.e., the it was not
necessary to send any signal).

Fix this by removing the breakpoint before resuming the process. This
ensures the process can run unobstructed.

After this, the test passed 200 consecutive runs successfully for me,
even while the system was under heavy load.

llvm-svn: 349491