Bug 1541189 - Moved test to tier 3 - r=whimboo

We'll promote it to tier 2 once we have fixed the intermittents

Differential Revision: https://phabricator.services.mozilla.com/D26081

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

taskcluster/ci/test/marionette.yml

@@ -53,7 +53,7 @@ marionette-media:
     treeherder-symbol: MnM
     max-run-time: 5400
     instance-size: default
-    tier: 2
+    tier: 3
     mozharness:
         extra-options:
             - --test-manifest=stream-tests.ini