tox.ini: Measure test coverage

```
$ tox -e cover
...
cover runtests: commands[1] | coverage report --show-missing
Name                 Stmts   Miss  Cover   Missing
--------------------------------------------------
junit_xml/__init__     137      0   100%
tests                  198      0   100%
--------------------------------------------------
TOTAL                  335      0   100%
___________________________________________________________________________________ summary ____________________________________________________________________________________
  cover: commands succeeded
  congratulations :)
```