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)
```
We are now https://github.com/hpcugent/vsc-base/pull/140 is merged in.