Bug 28288: Prevent XSISBN.t to fail if service is returning 500

Bug 28288: Prevent XSISBN.t to fail if service is returning 500

16:00:45 koha_1       | WARNING: URL Request Failed 500 read timeout
16:00:45 koha_1       | WARNING: URL Request Failed 500 read timeout
16:00:45 koha_1       | WARNING: URL Request Failed 500 Can't connect to www.librarything.com:443
16:00:45 koha_1       | # Looks like you planned 6 tests but ran 5.
16:00:45 koha_1       | [13:59:51] t/db_dependent/XISBN.t
16:00:45 koha_1       | Dubious, test returned 255 (wstat 65280, 0xff00)
16:00:45 koha_1       | Failed 1/6 subtests
16:00:45 koha_1       | 	(less 3 skipped subtests: 2 okay)

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