Write tests for improvers base class

Improvers need to be tested. There are no tests as of now for
```
- vulnerabilities/improvers.py
- vulnerabilites/improve_runner.py
```

Useful: https://github.com/nexB/vulnerablecode/blob/ed2131656b1b3030c2f9eb28e25c10dbbedc8e1d/vulnerabilities/tests/test_import_runner.py