Apply focus before filtering for platform/environment support.

Focus is no longer ignored in cases where focused tests wouldn't run--now, no tests run. This makes it easier to reason about what will happen when you focus a test: no unfocused tests will run.
It's to try to break the comment over multiple lines/interleave it with the code, you can see with the ellipses below `*hangs head sheepishly*`... suggestions for how I might improve it?