Bug 1503756 - Change name of media device; r=achronop

Regenerating the ubuntu1604-test Docker image pulls in a new
package version that changes the name of a media device. This media
device name is currently hard coded in the mochitest test harness.

This commit makes a change to force regeneration of the Docker image
and updates the hardcoded device name to reflect the new version.

Differential Revision: https://phabricator.services.mozilla.com/D11927

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

taskcluster/docker/desktop1604-test/Dockerfile

@@ -1,7 +1,6 @@
 FROM          ubuntu:16.04
 MAINTAINER    Joel Maher <joel.maher@gmail.com>
 
-
 RUN mkdir /builds
 RUN useradd -d /builds/worker -s /bin/bash -m worker
 WORKDIR /builds/worker

testing/mochitest/runtests.py

@@ -777,7 +777,7 @@ def findTestMediaDevices(log):
         return None
 
     # Hardcode the name since it's always the same.
-    info['audio'] = 'Monitor of Null Output'
+    info['audio'] = 'Monitor of Dummy Output'
     return info