tox.ini: Measure test coverage

```
$ tox -e cover
...
cover runtests: commands[1] | coverage report
Name                 Stmts   Miss  Cover
----------------------------------------
junit_xml/__init__     137     14    90%
tests                  162      1    99%
----------------------------------------
TOTAL                  299     15    95%
_________________________________________________________________________________________________________________________________________________________ summary __________________________________________________________________________________________________________________________________________________________
  cover: commands succeeded
  congratulations :)
```