Feature/fix exptest

Fixed some bugs found when writing document for exptest samples.

The goal was to define a exptest function with multiple paths like:

```python
def exptest(test):
    test.add("tests samples/vowpal")
```

where `tests` and `samples/vowpal` both are directories containing unit test files.

The first problem was that when `tests` is first specified for a directory, it throws an exception when adding tests in another directory: `samples/vowpal`. This is because of `unittest.defaultTestLoader.discover`'s behavior around `top_level_dir`, which could be solved by specifying this variable whenever calling this method.

The second problem was tests in `samples/vowpal` were not executed. This is because the old name of test file in this directory is `vowpal_util_test.py` but in default `discover` method searches only for files matching `test*.py`.