add avail_subclasses function + unit test

ported from `avail_easyblocks` method in `easybuild/tools/options.py`, required for https://github.ugent.be/hpcugent/easytest/pull/1

let's see what @stdweird has to say about this one, because I'm fairly sure he's the original author ;)
it's already there because the `FrozenDictKnownKeys` class has its own `self.log` fancylogger

I'll add the module logger and use it