Bug 14204: Fix t/db_dependent/Labels/t_Batch.t failing test from Bug 12991

Bug 14204: Fix t/db_dependent/Labels/t_Batch.t failing test from Bug 12991

This patch updaes the batch_id variable after items are added to test batch

To test:
1. prove t/db_dependent/Labels/t_Batch.t and see two tests fail
2. apply patch
3. prove again, tests pass!

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
NOTE: The $batch->add_item() call to C4::Creators::Batch::add_item
      triggers the change of the batch_id so this line is necessary!
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>