message: [test] Make sure PROMPT is a string
  Currently tests using expect_prompt are failing on the Python 3 bot with
  an error saying "argument must be str, not bytes". I don't have a Python
  3 build handy, but I suspect this might fix that.
diff: @@ -22,7 +22,7 @@ else:
  class PExpectTest(TestBase):
  NO_DEBUG_INFO_TESTCASE = True
  - PROMPT = "(lldb) "
  + PROMPT = str("(lldb) ")
  def expect_prompt(self):
  self.child.expect_exact(self.PROMPT)
  