Prioritize test queue by comparing open scope depth

[tolmasky]

Right now tests are run in the order they are reached in our tree traversal. This can be suboptimal if a bunch of tests are queued while a serial test with an open scope is running, because then the next test with access to that scope will be placed in the back of the queue. This will maximize browser starvation, along with just keeping around a lot more garbage than we need. We should decide which test to do next by how many open scopes it has.

[tolmasky]

Perhaps prioritize first by open scope depth with browsers, and secondly by absolute depth.