add unit test suite, include unit test for broken is_dir method

depends on https://github.com/hpcugent/vsc-base/pull/140

only includes a single unit test for now, but this is already a nice example of why it's interesting to have them:

```
PYTHONPATH=$HOME/work/vsc-base/lib:$PWD/lib:$PYTHONPATH python test/posix.py
F
======================================================================
FAIL: test_is_dir (__main__.PosixTest)
Tests for is_dir method.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/posix.py", line 37, in test_is_dir
    self.assertEqual(self.po.is_dir(os.environ['HOME']), True)
AssertionError: None != True

----------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (failures=1)
```
so these tests fail?
anyway @itkovian i guess it is not used anywhere, so we can better remove the method? (there was another one too that does nothing)
at the very least the empty method should raise NotImplenented; this is indeed too dangerous to have dangling around