Bug 15150: Make t/ tests skip if Test::DBIx::Class absent

Bug 15150: Make t/ tests skip if Test::DBIx::Class absent

Tests in t/ should always pass for building the Debian packages
for Koha. But we've started using Test::DBIx::Class for writing
mocked tests, and that lib is not (yet) packaged fro Debian 7+.

This means build is failing. Devs and jenkins use the lib from CPAN.

This patch makes the tests skip if the lib is absent.

To test:
- Install Test::DBIx::Class
  $ sudo cpanm Test::DBIx::Class
- Run the tests:
  $ prove t/
=> SUCCESS: Tests pass
- Uninstall Test::DBIx::Class
  $ sudo cpan -U Test::DBIx::Class
- Run the tests:
  $ prove t/
=> SUCCESS: Tests still pass (those needing the lib are skipped)
- Sign off :-D

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Works as advertised. All test pass successful

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>