Bug 12782: (followup) skip tests that fail because of provider's error

Bug 12782: (followup) skip tests that fail because of provider's error

It seems that XISBN is failing sometimes, if you run the tests a couple
of times in a row. An error 500 is raised by lwp.

This problem should be trapped and related tests skipped. Because too much
noise could make people pay no attention to this tests failing.

To reproduce:
- Run the tests several consecutive times:
  $ prove -v t/db_dependent/XISBN.t
=> FAIL: XISBN test will fail eventually, printing a networking-related warning.
- Apply the patch
- Repeat the test
=> SUCCESS: when XISBN the networking/connection refused problem arises, the test
  is skipped, and it still passes.

Regards
To+

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>