Bug 37898: Wrap Relationships.t inside a transaction

Bug 37898: Wrap Relationships.t inside a transaction

This tests leave data on the DB and need to be fixed. This patch does
that.

To test:
1. Run:
   $ ktd --shell
  k$ echo "SELECT COUNT(*) FROM borrowers \G" | koha-mysql kohadev
  k$ echo "SELECT COUNT(*) FROM borrower_relationships \G" | koha-mysql kohadev
2. Run:
  k$ prove t/db_dependent/Patron/Relationships.t
=> SUCCESS: Tests pass
3. Repeat 1
=> FAIL: Numbers don't match the original ones!
4. Apply this patch
5. Repeat 2 and 3
=> SUCCESS: Tests still pass!
=> SUCCESS: Numbers match!
6. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>