tests: lwm2m_rd_client: Added more fff tests

If the rdclient is suspended or deregistered any network errors caused
by lwm2m_engine and forwarded by socket_fault handler should not change
the rd client state.

Signed-off-by: Andreas Chmielewski <andreas.chmielewski@grandcentrix.net>
Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>