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.
I'd just one line it to "// now filter the tests to run to those focused, if any...". I think the rest of the `// ... comment ...` formatted comments below follow just fine after that. 

But, then again, as I prefaced this is `SUPER NIT`.