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 ;)
oneliner of the day (without verifying)
```python
return [res.group('modname') for res in map(module_regexp.match, os.listdir(pkg_path)) if res]
```