tests: lwm2m: Fix minor timing issues on tests

* Wait for update when new object is created

  Sometimes our blockwise transfer starts before server have
  received an LwM2M Update. This causes Leshan to reject the SEND
  because is was not aware of object /19/0

* Wait "registered" message already in conftest.py.
  Sometimes Qemu boots faster, and the testcase don't see the
  log.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>