Bug 35536: Reorganize Plugins.t

Bug 35536: Reorganize Plugins.t

Move stuff in the middle into new subtest.
Add transaction around second set of subtests.

Test plan:
t/db_dependent/Koha/Plugins/Plugins.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit e30915eb6b9266d5e1caca528d9ff0650b966253)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>