Bug 19605: (QA follow-up): available_backends fix reduce

Bug 19605: (QA follow-up): available_backends fix reduce

available_backends was only considering dir backends for reduce, it should consider all backends, including plugin backends.

This relates to the ILLOpacbackends syspref, where if you add a backend name that happens to be a plugin, it was not disabling it for the OPAC.
This change considers all backends for Illopacbackends, regardless of if its a plugin or a directory backend.

This was made evident while writing tests for bug 36197.
Regression tests for this particular use case, and others, are part of bug 36197.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>