Bug 31889: (follow-up) Add test for explicit warning

Bug 31889: (follow-up) Add test for explicit warning

On bug 29697, an explicit call to 'warn' was added, but no tests were
added for that behavior.

This patch adds that.

To test:

1. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Biblio/Metadata.t
=> FAIL: There's a warn, no tests for it :-(
2. Apply this patch
3. Repeat 1
=> SUCCESS: A new test was added, no more warns printed
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>