Bug 1416747 - [marionette] Hardening test_cli_arguments.py TestCommandLineArguments.test_startup_timeout. r=ato

Using a startup timeout of 0s should always cause an IOError,
even for builds which startup within 100ms (pgo, Nightly opt).

Further the test should not modify the internal startup timeout
property, but pass the timeout as parameter to "start_session()".

Differential Revision: https://phabricator.services.mozilla.com/D10224

--HG--
extra : moz-landing-system : lando

testing/marionette/harness/marionette_harness/tests/unit/test_cli_arguments.py

@@ -37,16 +37,10 @@ class TestCommandLineArguments(MarionetteTestCase):
             self.assertTrue(safe_mode, "Safe Mode has not been enabled")
 
     def test_startup_timeout(self):
-        startup_timeout = self.marionette.startup_timeout
-
-        # Use a timeout which always cause an IOError
-        self.marionette.startup_timeout = .1
-        msg = "Process killed after {}s".format(self.marionette.startup_timeout)
-
         try:
             self.marionette.quit()
-            with self.assertRaisesRegexp(IOError, msg):
-                self.marionette.start_session()
+            with self.assertRaisesRegexp(IOError, "Process killed after 0s"):
+                # Use a small enough timeout which should always cause an IOError
+                self.marionette.start_session(timeout=0)
         finally:
-            self.marionette.startup_timeout = startup_timeout
             self.marionette.start_session()