Coverage XML sometimes checked too early

I could have missed something so my apologies if there is an easier fix but I have noticed that depending on the order I install pytest-azurepipelines or
pytest-cov via pip that sometimes the pytest11 plugin hook invoke coverage XML
production after the pytest-azurepipeline session finish hook, meaning it
thinks the coverage XML is not present. This is less a problem when using pip but if using pipenv it can be more difficult to control install order.

Output observed - skip upload appears before coverage report:

```
##[warning]Coverage XML was not created, skipping upload.

---------- coverage: platform linux2, python 2.7.15-final-0 ----------
```