Bug 1470397 - Use logger from created from |mach test| in testing/firefox-ui/mach_commands.py, r=whimboo

This will allow |mach test| to run firefox-ui tests.

Differential Revision: https://phabricator.services.mozilla.com/D1844

testing/firefox-ui/mach_commands.py

@@ -77,8 +77,10 @@ def run_firefox_ui_test(testtype=None, topsrcdir=None, **kwargs):
         kwargs['tests'] = [os.path.join(fxui_dir, 'tests', test)
                            for test in test_types[testtype]['default_tests']]
 
-    kwargs['logger'] = commandline.setup_logging('Firefox UI - {} Tests'.format(testtype),
-                                                 {"mach": sys.stdout})
+    kwargs['logger'] = kwargs.pop('log', None)
+    if not kwargs['logger']:
+        kwargs['logger'] = commandline.setup_logging('Firefox UI - {} Tests'.format(testtype),
+                                                     {"mach": sys.stdout})
 
     args = Namespace()