Allow -s to run single tests

When you use the `-s` option, you can name a particular test suite or a prefix of test suites (e.g. `-sdiff::tree` or just `-sdiff`). This PR extends that so that the string after the `-s` is passed all the way through to the individual test suite runner and is applied as a filter on individual tests to run. This allows you to go all the way to `-sdiff::tree::merge` to run single tests if desired.