Support running only a subset of the test suite

This small update gives meaning to command-line arguments to the `specify` command. (AFAIK it only looks at -e and -r options).

If arguments are given to `specify` then it will assume they are all subdirectories in test/ (or tests/) and it will only run tests in those subdirectories.

I did not see any tests for the binary runner, so this patch has no unit tests accompanying it. I also did not see this test runner mentioned in the README, so there are no documentation changes either.