eldbus: add test suite for eldbus (eldbus_signal_handler*)

eldbus: add test suite for eldbus (eldbus_signal_handler*)

Summary:
add tests for next API:
eldbus_signal_handler_add()
eldbus_signal_handler_del()
eldbus_signal_handler_connection_get()
eldbus_signal_handler_interface_get()
eldbus_signal_handler_member_get()
eldbus_signal_handler_path_get()
eldbus_signal_handler_sender_get()
eldbus_signal_handler_ref()
eldbus_signal_handler_unref()
eldbus_signal_handler_free_cb_add()
eldbus_signal_handler_free_cb_del()

Reviewers: cedric, NikaWhite, myoungwoon, raster, artem.popov

Subscribers: jpeg

Differential Revision: https://phab.enlightenment.org/D4910

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>