Bug 30195: Search by ISBN if it is provided in suggestion

Bug 30195: Search by ISBN if it is provided in suggestion

When a patron enters an ISBN/ISSN when suggesting a new purchase, the
ISBN is used to find duplicates. Title/Author are ignored (as patrons
might misspell them, and the ISBN provides a better way to find
duplicates)

Test Plan:
* in the OPAC, go to /cgi-bin/koha/opac-suggestions.pl
* Click "new purchase suggestion"
* Enter any title
* Enter an ISBN that exists in your library
* Click "Submit your suggestion"
-> A new purchase suggestion has been submitted

* Apply the patch!

* Again start a new purchase suggestion, enter any title and the
  duplicate ISBN, and Submit
* You should see the note "A similar document already exists: ..."

Please note that the title should not be required when entering an ISBN,
but as the list of required fields is managed via OPACSuggestionMandatoryFields
I fear that it's rather complicated to make title an optional required
field if isbn is provided.

I also added a simple non-DB test case to t/db_dependent/Suggestions.t
(in a subtest at the end), but could not actually run it as I haven't
gotten around to set up / try a testing Koha dev env...

Sponsored-by: Steiermärkische Landesbibliothek
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>