Add ability to use FOCUS environment variable to focus tests

We found that our CI test runs were getting really long and wanted to split them up into multiple jobs, so we came up with this and have been using it for 3 months without any issues. It has the same behaviour as prepending tests with `focus_`, but it works using a `FOCUS` environment variable on the Integration Tests scheme, which can then be set for each CI job with `subliminal-test [...] -e FOCUS testFoo,testBar`.

We couldn't figure out a way to write tests for this, given that it's checking the environment variables for the list of tests to run, but let me know if you can think of something.