setup.py: add a 'testing' extra
setup.py: add a 'testing' extra

this is conventional among python packages to have an 'testing' extras_require
so developers can bootstrap a testing environment easily with something like:

pip install -e .[testing]

The pytest-cov is a pluging to integrate coverage.py with pytest.

pytest-randomly is to shuffle tests in a random order to check against inter-test
dependencies

googlefonts/fonttools,fonttools/fonttools