Bug 17909: Add tests for merging with another authtype

Bug 17909: Add tests for merging with another authtype

Originally aimed for bug 9988. Adjusted in line with other subtests.
Will polish the three subtests a little more on a follow-up.

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>