Bug 905775 - Firefox is failing silently in recent travis builds during `make test-agent-test` step

This turns off the desktop extensions for the tests, and enabled the touch
events in DEBUG mode, so that the tests can actually use touch events.
Merge pull request #11639 from julienw/fix-travis-2

Bug 905775 - Firefox is failing silently in recent travis builds during `make test-agent-test` step
r=ochameau