tests lwm2m_rd_client: Allow only on native_posix

This test is too dependent on the exact order of events,
and known to fail in most platforms but native_posix.
To mitigate the issue and avoid blocking development
due to failed CI tests, let's only allow the test in
native_posix.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>