Bug 1600332 - Fix awsy tests to measure common case non-triplets new tab page r=Mardak

Differential Revision: https://phabricator.services.mozilla.com/D55530

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

testing/awsy/conf/prefs.json

@@ -10,5 +10,5 @@
     "plugin.disable": true,
     "startup.homepage_override_url": "",
     "startup.homepage_welcome_url": "",
-    "trailhead.firstrun.branches": "join"
+    "trailhead.firstrun.branches": "join-none"
 }

testing/awsy/conf/tp6-prefs.json

@@ -12,5 +12,5 @@
     "plugin.disable": true,
     "startup.homepage_override_url": "",
     "startup.homepage_welcome_url": "",
-    "trailhead.firstrun.branches": "join"
+    "trailhead.firstrun.branches": "join-none"
 }