service: fix lcore stop

service: fix lcore stop

lcore_states store the state of the lcore. Fixing the invalid
dereference of lcore_states with service number

Unit test case service_lcore_start_stop fails with the above fix.
Service core was stopped without stopping the service.

This commit fixes the test by adding negative and positive cases of
stopping the service lcore before and after stopping the service
respectively

Fixes: 21698354c832 ("service: introduce service cores concept")
Fixes: f038a81e1c56 ("service: add unit tests")

Signed-off-by: Guduri Prathyusha <gprathyusha@caviumnetworks.com>
Reviewed-by: Harry van Haaren <harry.van.haaren@intel.com>