Bug 7724: Skip Linker test if Zebra isn't set up

Bug 7724: Skip Linker test if Zebra isn't set up

One of the tests in t/db_dependent/Linker_FirstMatch.t ony works if Zebra is
running and authorities have been indexed. This was causing Jenkins agitas.
The test should be skipped if Zebra isn't running or authorities have not been
indexed at the time the test is run.

Signed-off-by: Ian Walls <koha.sekjal@gmail.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>