Unit testing for easyconfigs

This introduces unit tests for easyconfigs.

With _n_ easyconfig files, _n+1_ tests will be run, i.e. each easyconfig file will be parsed and an easyblock instance will be created for it, and a full dependency graph is built.

Example run:

```
$ python -m easyconfigs_test.suite
WARNING: xmlrunner module not available, falling back to using unittest...

.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
----------------------------------------------------------------------
Ran 953 tests in 55.632s

OK
```