Bug 16502: Add additional test to Plugins.t

Bug 16502: Add additional test to Plugins.t

In order to verify if the delete now really works, we add one test
in Plugins.t.

Test plan:
[1] Run the test.
[2] Bonus: Comment line 63 in Plugins.t where delete is called.
    Run the test again. It should fail now.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 959d69fa0107423ed31e20f4a6afb46d1e5c771c)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>