Bug 16868: Silence error t/db_dependent/Linker_FirstMatch.t

Bug 16868: Silence error t/db_dependent/Linker_FirstMatch.t

When the auth_header table has records which exclude 1xx and
2xx tags, the $bibfield doesn't match anything. This in turn
sets it to undef, which triggers an error on the next line
killing the test.

This was completely refactored to provide the data necessary
for the tests to pass, and to be more comprehensive (both MARC
and UNIMARC are tested). The tests are then run.

C4::Headings::authorities is mocked, so that this test is not
dependent on a search engine.

TEST PLAN
---------
1) back up DB
2) DELETE FROM auth_header;
3) SOURCE auth_header.sql;
   -- the provided file
4) prove t/db_dependent/Linker_FirstMatch.t
   -- should barf before running all the tests
5) apply all patches
6) prove t/db_dependent/Linker_FirstMatch.t
   -- should work happy
7) run koha qa test tools
8) restore your backup

Followed test plan, behaves as expected.
Signed-off-by: Marc <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>