tox.ini: Tweak test discovery

Replace `tests/` in `addopts` with `norecursedirs`

Better to filter than to add in a default, so as not to force folks to
always run all the tests.
Merge pull request #297 from msabramo/1.0-alpha__tox.ini_take_tests_out_of_pytest_addopts

tox.ini: Tweak test discovery