Bug 11543: Increasing test coverage for Category.pm

To test:
1. Run prove t/Category.t
2. Apply patch
3. Run prove t/db_dependent/Category.t - Notice 2 tests were run

Signed-off-by: daniel <daniel.barker.nz@gmail.com>
Made the requested changes.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 11543: (followup) add one more test

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>