Hide OPAC-invisible holdings from SRU/Z39.50

Hide OPAC-invisible holdings from SRU/Z39.50

The basic_holdings Supercat method filtered out deleted call numbers and
copies, but didn't filter out copies based on the OPAC visibility status
of copy location / status / the copy itself. This has undesired results
when third-party services request copies via SRU or Z39.50 and expose
copies that should not be visible to the public.

We wrap all of the visibility and deletedness checks for a given copy up
into a subroutine so that we can avoid repeating ourselves in the
basic_holdings logic. Also, if we missed a test, we can add it in one
handy place :)

(Thanks to Galen Charlton for finding two missing tests: circ lib
visibility and owning lib visibility!)

Signed-off-by: Dan Scott <dscott@laurentian.ca>