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 ;)
you mean a `log = fancylogger.getLogger(fname=True)` in only this function, or adding a global `_log` in the module on top?