Bug 37898: Wrap Indexer.t inside a transaction

Bug 37898: Wrap Indexer.t inside a transaction

This tests leave new data on the DB. This patch fixes it.

To test:
1. Run:
   $ ktd --shell
  k$ echo "SELECT COUNT(*) FROM biblio \G" | koha-mysql kohadev
2. Run:
  k$ prove t/db_dependent/Koha/SearchEngine/Elasticsearch/Indexer.t
=> SUCCESS: Tests pass
3. Repeat 1
=> FAIL: Biblio count raised
4. Apply this patch
5. Repeat 2 and 3
=> SUCCESS: Tests pass
=> SUCCESS: Biblio count remains
6. Sign off :-D

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