Bug 28303: Fix plugins system with multiple pluginsdir settings

Bug 28303: Fix plugins system with multiple pluginsdir settings

C4/Templates::badtemplatecheck mucks with the config('pluginsdir') array ref.
This makes sure it operates on a copy of the array.

To test:
   1) $ prove t/db_dependent/Templates.t

Signed-off-by: David Nind <david@davidnind.com>
JK: Fix commit message styling and add test plan

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>