tests: fix pytest API "surprise" in skipping tests

tests: fix pytest API "surprise" in skipping tests

pytest.mark.skipif apparently iterates through a class's methods,
applying itself onto the various methods.  Now, since we're deriving
from a parent class, the method is actually the same object inherited
from the parent, so the decorator will apply itself on the parent's
testrunning method (test_refout).  The result is that any TestRefout
tests after "test_commands.py" will be skipped...

This only became apparent after adding ospf6d/test_lsdb.py;  before,
test_commands.py was the last test in the list so it didn't matter...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>