Bug 18976: Fix t/db_dependent/Auth.t cleanup

Test t/db_dependent/Auth.t seems to have an ineffective test data cleanup.
Data generated by TestBuilder is left in borrowers, branches, categories,
sms_providers and sessions tables after the test.

To replicate:
1. Check the row count of borrowers, branches and categories tables
2. prove t/db_dependent/Auth.t
3. Repeat step 1
4. Observe borrowers the following tables have increased in row count:
   - borrowers
   - branches
   - categories
   - sessions
   - sms_providers

To test:
1. Before applying the patch, go through steps at "To replicate" plan
2. Apply patch
3. Go through steps at "To replicate" plan
4. Observe step 4 no longer applies and those tables have the same number of
   rows as before executing the test.

This issue has been happening in REST tests as well, and this solution is
directly copy-pasted from t/db_dependent/api/v1/cities.t

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

t/db_dependent/Auth.t

@@ -29,6 +29,10 @@ my $schema  = Koha::Database->schema;
 my $builder = t::lib::TestBuilder->new;
 my $dbh     = C4::Context->dbh;
 
+# FIXME: SessionStorage defaults to mysql, but it seems to break transaction
+# handling
+t::lib::Mocks::mock_preference( 'SessionStorage', 'tmp' );
+
 $schema->storage->txn_begin;
 
 subtest 'checkauth() tests' => sub {