core: Use @Before in searchbackend tests

Added an set up method (with @Before) to initialize the object being
tested in test cases that have more than one test, as per proper JUnit4
conventions.

Change-Id: Id3a1ca0aca1f570bf2763d6991d56427d0c17764
Signed-off-by: Allon Mureinik <amureini@redhat.com>