Move test requirement coverage into tox.ini

Like commented in the test-requirements.ini file the test requirement
coverage can be moved into the tox.ini file when tox>=1.4 is required.
The minimum version at the moment is 1.6.

Change-Id: Ic71f63fb0fab0470e4732099bb8ab5c13dd98538
Merge "Move test requirement coverage into tox.ini"