Bug 19979: Fix get_facets_info test in Search.t

Bug 19979: Fix get_facets_info test in Search.t

The test should remove the holdingbranch key in the expected results when
there is only one branch.
See the logic in C4::Koha.

Test plan:
[1] Run Search.t on a database with multiple branches.
[2] Run Search.t on a database with one branch. Without this patch the test
    would fail: Failed test '_get_facets_info returns the correct data'.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>