samples: rpmsg_service: do not run test for nrf5340 in CI

We cannot automatically flash both master and remote images
to nRF5340 application and network core, this needs to be
done separately. We therefore, cannot execute the test on
nRF5340, so we switch to make the test build-only on this
platform. We also make nrf5340_cpuapp an integration
platform for the given sample.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>