New: launch only one test from a classTest

Previously it wasn't possible to launch only one test from a file.
Now OJUnit has this feature, the command to do that is "ojtest NAME_OF_THE_FILE:NAME_OF_THE_TEST"
This method has a strange name. If `_singleTestName` is nil, `isSingleTestName:` will return `YES` for any selector.

I'm guessing this method should actually be named something like, `selectorMatchesTestPattern:(SEL)selector`, a function which checks the selector matches in single test mode, or matches all selectors in regular mode.

In fact, _singleTestName could be replaced by a more general concept, `selectorRegex` or something and be set to `.*` by default, and to a specific test name in single test mode. This way the code is kind of obvious, yet flexible. You could use the same code to match all selectors, a specific selector or a certain subset of selectors.