Bug 28303: Add unit test for badtemplatecheck

Bug 28303: Add unit test for badtemplatecheck

This tests that pluginsdir value is not modified by the call to
badtemplatecheck to make sure badtemplatecheck operates with its own
copy of pluginsdir configuration value.

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit d329457a02271e2f43446075d37c33ce534a2969)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>