Deprecate broken 'setup.py test' integration

Just demand use of 'tox' instead.  Ideally 'setup.py test' would work
smoothly but on account of a couple of problems with program arguments
and plugin discovery, expeditiously kill off the half-working thing
instead.
Install coverage pytest plugin by default

Note the plugin is installed, not used all the time -- it slows down
test execution by quite a bit.  Yet, I have found in practice not
having the '--cov' option at hand makes me less likely to use coverage
in simple cases, so just make it installed by default to put it close
at hand.
Add test coverage reporting instructions
Fix rst warning about too-short title underline