Bug 1434293 - Move mochitest off deprecated Marionette prefs. r=gbrown

The marionette.logging preference has been deprecated for some time.
We will want to use the official marionette.log.level preference instead.

Supported preferences are listed in
testing/marionette/prefs/marionette.js.

MozReview-Commit-ID: 4Z6OgqITe14

--HG--
extra : rebase_source : 62eabd701e79002b3828c3452e634966a2846643

testing/mochitest/runtests.py

@@ -1829,9 +1829,7 @@ toolbar#nav-bar {
 
         # Enable tracing output for detailed failures in case of
         # failing connection attempts, and hangs (bug 1397201)
-        options.extraPrefs.append(
-            "marionette.logging=%s" %
-            "TRACE")
+        options.extraPrefs.append("marionette.log.level=%s" % "TRACE")
 
         # get extensions to install
         extensions = self.getExtensionsToInstall(options)