Investigate if we should look at prioritizing API test-requests over Web-UI ones

See https://www.webpagetest.org/forums/archive/index.php?thread-12163.html

While we're scaling up capacity, refining metrics, etc., it'd be nice to ensure that our many tests get the highest priority.

Example: batch of 50 API tests hits the WebPageTest server, but a few teams are diligently looking into some regression issue, causing extra load/queuing of tests.