slapos.cookbook/testing: fix missing version pin for mock

Because we run egg tests with setup.py test, which installs missing
eggs, if an egg was not installed by buildout, then it installed before
running test. This was the case for mock, which is now python3 only (
https://pypi.org/project/mock/4.0.0b1/ ) and we started to see test
failures.

To solve this issue, refactor the setup definition to use
extra_requires, which seems to work fine in buildout now. Keep
test_requires because it's the what `python setup.py test` uses.

Clean buildout profiles to install slapos.cookbook[test] for test
instead of duplicating the content of test_requires.

/reviewed-on nexedi/slapos!690