[PATCH] tox: Omit tests and manage.py when running coverage tests

[Finucane, Stephen]

> Having the tests in the coverage reports artifically improve the
> coverage percentage, because every line in tests is being run.
> 
> Signed-off-by: Damien Lespiau <damien.lespiau at intel.com>

Good point.

Reviewed-by: Stephen Finucane <stephen.finucane at intel.com>