Add a keyword argument 'endstr' to TestBase.expect() method to assert that the output
will end with 'endstr'.

Add TestBase.switch_to_thread_with_stop_reason(stop_reason) to select the thread with
the stop reason = 'stop_reason' as the current thread.

o TestWatchLocation.py:

Modified to switch to the stopped thread with stop reason = watchpoint and to evaluate
an expression with expected output for stronger assertion.

llvm-svn: 140890