Use 'desktop-test-large' worker instance for taskcluster tasks (#812)

[whimboo]

Fix for issue #812.

It changes the worker type to 'desktop-test-large' for both the functional and update tests. @djmitche, mind reviewing it?

[whimboo]

I triggered an example task here: https://tools.taskcluster.net/task-inspector/#WW_mn3EqTZKyRGfZHdRV1g/

[djmitche]

r+