tests: don't rely on pylint command being available
This is a replay of d5c53373c9fe, accidentally left out in f25542a8833d: Relying on the module's presence is much more reliable, in particular when using MacPorts.
This is a replay of d5c53373c9fe, accidentally left out in f25542a8833d: Relying on the module's presence is much more reliable, in particular when using MacPorts.