Bug 17913: Do not keep a cleared subfield in loose merge mode

Bug 17913: Do not keep a cleared subfield in loose merge mode

If you modify an authority and clear a specific subfield, you expect that
merge respects your edit and clears this subfield too in the biblio
records. It does in the new strict mode, but it does not yet in the
default loose mode.

This patch fixes that by adjusting the code around $exclude so that it
uses a new hash skip_subfields, built from the reporting tags from the old
and the new authority record.

This is supported again by some changes in the unit test.

Test plan:
Run t/db_dependent/Authorities/Merge.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>