Bug 17950: Merge.t: Improve variable handling at top level

Bug 17950: Merge.t: Improve variable handling at top level

This patch does:
[1] Remove unused $dbh.
[2] Move all mocking statements to one sub.
[3] Promote a few lexicals to globals when used in the subtests.

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

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>