Bug 21671: Prevent Modifications.t to fail randomly

Bug 21671: Prevent Modifications.t to fail randomly

If repeatable is set to 0 by TestBuilder, the tests will fail with:
t/db_dependent/Koha/Patron/Modifications.t .. 1/6     # Looks like you planned 20 tests but ran 9.

 #   Failed test 'approve tests'
 #   at t/db_dependent/Koha/Patron/Modifications.t line 259.
[repeatable not set for attribute type and tried to add a new attribute for the same code]# Looks like your test exited with 255 just after 5.

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>