test/service: fix race condition on stopping lcore

test/service: fix race condition on stopping lcore

There is a potential race condition in 'service_attr_get' which will cause
test failures since the service core thread is still running while the
values are being retrieved/reset.

This patch fixes the race condition by waiting for the service core thread
to stop before continuing with the unit test checks.

Fixes: 4d55194d76a4 ("service: add attribute get function")

Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>