Make sure we don't try to remove device forwards that no longer exist

The stop() function is apparently being called multiple times at the end of a test run, causing us to sometimes attempt to remove device forwards that were removed in an earlier stop() attempt but before the conditions checked in stop() to remove the device forwards get changed. This caused a traceback to be printed out at the end of every test run, complaining about attempts to remove device forwards that don't exist.

Rather than dig through to figure out why/where stop() is getting called twice, this patch just papers over the issue by making sure there is at least one device forward still listed before attempting to remove them.

Differential Revision: https://phabricator.services.mozilla.com/D9175

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1499889
gecko-commit: 6ff410edad90dbc51d72aeb722d0ff8d182606c1
gecko-integration-branch: autoland
gecko-reviewers: gbrown

tools/wptrunner/wptrunner/browsers/fennec.py

@@ -227,8 +227,9 @@ def stop(self, force=False):
         if self.runner is not None:
             try:
                 if self.runner.device.connected:
-                    self.runner.device.device.remove_forwards(
-                        "tcp:{}".format(self.marionette_port))
+                    if len(self.runner.device.device.list_forwards()) > 0:
+                        self.runner.device.device.remove_forwards(
+                            "tcp:{}".format(self.marionette_port))
             except Exception:
                 traceback.print_exception(*sys.exc_info())
             # We assume that stopping the runner prompts the