tests: lwm2m: Add Qemu X86 and fix Qemu Cortex-M3

* Add support for running interoperability tests with
  Qemu X86.
* Remove some debugging messages to allow binary to
  fix Qemu Cortex-M3 board.
* Tune buffer and stack sizes to fit all boards.

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