clar: refactor explicitly run test behavior Previously, supplying `-s` to explicitly enable some test(s) would run the tests immediately from the argument parser. This forces us to set up the entire clar environment (for example: sandboxing) before argument parsing takes place. Refactor the behavior of `-s` to add the explicitly chosen tests to a list that is executed later. This untangles the argument parsing from the setup lifecycle, allowing us to use the arguments to perform the setup.