Cool. I tried adding a test for this, but if fails with an ImportError.

I might be misunderstanding how file paths are supposed to work.

```python
def test_file_path(self):
    count = DiscoverRunner().build_suite(
        ["test_discovery_sample/tests_sample.py"],
    ).countTestCases()

    self.assertEqual(count, 3)
```

Here's the error:

```python
raise ImportError('Start directory is not importable: %r' % start_dir)
ImportError: Start directory is not importable: 'test_discovery_sample/tests_sample.py'
```