Bug 1500874 - Set telemetry-tests-client suite back to CI tier 3; r=Dexter

Differential Revision: https://phabricator.services.mozilla.com/D9370

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

taskcluster/ci/test/misc.yml

@@ -63,7 +63,7 @@ telemetry-tests-client:
     treeherder-symbol: tt(c)
     max-run-time: 5400
     checkout: true
-    tier: 2
+    tier: 3
     mozharness:
         script: telemetry/telemetry_client.py
         config: