Bug 16502: Adjust test for GetPlugins

Bug 16502: Adjust test for GetPlugins

The current test assumes that GetPlugins will return the test plugin
as the first one in the array. This is not correct.
This patch adjusts the test to a grep.

Test plan:
Run the test.
Bonus: Add additional plugins. Run the test again.

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 6ad5770786c6646ce68ffdfec9080645fc25772e)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>