Bug 37872: Consider enable_plugins = 0 when fetching plugin backends

Bug 37872: Consider enable_plugins = 0 when fetching plugin backends

This patch builds on top of Andreas patch checking for Koha::Plugins->new() instead as the enable_plugins is already done there.
It also adds a check in reply to Joubu's observation, which is indeed an additional bug.

Test plan:
1) Apply tests patch. Run the tests. Notice it fails
2) Apply the remaining patches. Run the tests. Notice they pass.

Additional (and better) tests can be written, but for now I think this is good enough.
Additional tests should be written on top of bug 36197 (which adds a dummy ILL backend plugin for testing), at:
https://bugs.koha-community.org/bugzilla3/page.cgi?id=splinter.html&bug=36197&attachment=171371

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>