This is so that I can run `tox -- -n 8` to run the tests in parallel, or
`tox -- tests/checker/test_misc.py::TestMisc::test_html5` to run just a
single test, without having to repeat all the other options.
I haven't moved --cov=linkcheck because I don't want coverage results
when I'm limiting the test run to a single test (they just make the
interesting bit -- the test result itself -- scroll up).
I've also added -ra to the default option list because then several
tests fail, I'd like to see a list of their names in one place, not
spead out between the huge tracebacks.
We must do this, because py.test adjusts sys.path to make
'tests.test_foo' importable [*]. When py.test does this, the
'linkcheck' directory at the top of the git tree is the one that gets
imported in the tests. If we've told pip to use develop mode, all's
fine. If we haven't, then we're going to get errors because extension
modules like _network.so get installed into
.tox/*/lib/*/site-packages/linkcheck/network and not into
./linkcheck/network/
[*] http://doc.pytest.org/en/latest/goodpractices.html#choosing-a-test-layout-import-rules
