runtests.sh doesn't check/auto install when `coverage` is missing

```
09:50:59  + bash runtests.sh --clean
09:51:00  PYTHONPATH: /home/jenkins/agent/workspace/Monai-pip/monai:
09:51:00  Python: /usr/bin/python
09:51:05  clean
09:51:05  /usr/bin/python: No module named coverage
```