explain how to run tests for a given plugin
explain how to run tests for a given plugin

See
https://meta.discourse.org/t/how-do-i-run-only-discourse-plugins-poll-spec/47244/5
for the corresponding discussion.

Signed-off-by: Loic Dachary <b6eae37fc74638590e3f9030895d42446505d41b@dachary.org>

natefinch/discourse,natefinch/discourse