Add new, small "Testing" section to CONTRIBUTING.rst

"Drive-through" contributor here. As in many other projects, one of the
most time-consuming step before starting to do any actual work is
finding in my bash history (!) the right tox/pytest/test_foo.sh
incantation. No surprise maintainers have to beg for test(s) all the
time ;-P

Signed-off-by: Chris Lamb <lamby@debian.org>