Bug 18996: Add checkin subtest in SIP/Message.t

Bug 18996: Add checkin subtest in SIP/Message.t

Variable $branch was not used.
Promoted some global vars to our.
Shared the branchcode between all three subtests now.
The third subtest contains all six cases mentioned in the first patch.

Test plan:
Run t/db_dependent/SIP/Message.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Colin Campbell <colin.campbell@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>