Intermittent Test Failure: verify reprocessing previews with a revision filter

This test has a flaw, and is timing out intermittently due to a large variance in how long the preview generating time can take for the gif image that is being used (I've had tests that complete in 4 seconds, and some that would complete in 40 seconds). The test log from the instance that happened recently showed that PP time took 30s.

The flaw in the test is that the preview is processed twice for the second revision. The `PreviewsAPI.enable` method only purges the queue the first time it is invoked (rightfully so), therefore the second revision was processed immediately upon `PreviewsAPI.enable`, and then once again when the reprocessAll task is invoked.

Purging the queue once more before we enable the PP should cut the test time in half, and probably eliminate the timeout.