Bug 1571624 - Switch awsy tests to measure common case non-triplets new tab page r=erahm

Differential Revision: https://phabricator.services.mozilla.com/D41752

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

testing/awsy/conf/prefs.json

@@ -10,5 +10,6 @@
     "network.proxy.type": 1,
     "plugin.disable": true,
     "startup.homepage_override_url": "",
-    "startup.homepage_welcome_url": ""
+    "startup.homepage_welcome_url": "",
+    "trailhead.firstrun.branches": "join"
 }

testing/awsy/conf/tp6-prefs.json

@@ -12,5 +12,6 @@
     "network.proxy.type": 1,
     "plugin.disable": true,
     "startup.homepage_override_url": "",
-    "startup.homepage_welcome_url": ""
+    "startup.homepage_welcome_url": "",
+    "trailhead.firstrun.branches": "join"
 }