Bug 16249: Zebra-specific tests should pass with ES disabled

Bug 16249: Zebra-specific tests should pass with ES disabled

This patch makes the t/db_dependent/Search.t tests correctly set
'Zebra' for the 'SearchEngine' syspref.

To test:
- On top of bug 12478 branch, run
  $ prove t/db_dependent/Search.t
=> FAIL: Tests fail
- Apply the patch
- Run:
  $ prove t/db_dependent/Search.t
=> SUCCESS: Tests now pass.

Signed-off-by: Chris <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>