ldb:test:api_search: use class method to add index

ldb:test:api_search: use class method to add index

The index attribute of these classes was a mutable object, which made
me nervous. If a subclass were to alter the class.index (rather than
replace it), it would affect tests in other subclasses.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>