make sure the share backend test case can find the correct interface

tests/lib/share/backend.php

@@ -19,6 +19,8 @@
 * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+OC::$CLASSPATH['OCP\Share_Backend']='lib/public/share.php';
+
 class Test_Share_Backend implements OCP\Share_Backend {
 
 	const FORMAT_SOURCE = 0;