Bug 1423918 - Run TVw as tier 2; r=me, a=test-only

TVw, test-verify-wpt, is just like TV/test-verify, but for web-platform
tests. It has been running as tier 3 for several weeks. Recent runs are
trouble free. See https://developer.mozilla.org/en-US/docs/Test_Verification

taskcluster/ci/test/web-platform.yml

@@ -137,7 +137,10 @@ test-verify-wpt:
             # do not run on ccov; see also the enable_code_coverage transform
             linux64-ccov/.*: []
             default: built-projects
-    tier: 3
+    tier:
+        by-test-platform:
+            windows10-64-asan.*: 3
+            default: 2
     mozharness:
         extra-options:
             - --verify