Bug 1768964 - Use setattr on Namespace object. r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D146133

testing/mochitest/mach_commands.py

@@ -189,7 +189,7 @@ class MochitestRunner(MozbuildObject):
         options = Namespace(**kwargs)
 
         # Disable fission until geckoview supports fission by default.
-        options["disable_fission"] = True
+        setattr(options, 'disable_fission', True)
 
         return runjunit.run_test_harness(parser, options)