tests: openthread: Add CONFIG_NET_TEST to make self contained

Add CONFIG_NET_TEST=y to make the test self contained and not
trying to create native_posix Ethernet interface which will
normally fail.
This prevents this error from printing

[00:00:00.000,000] <err> eth_posix: Cannot create zeth (-1)

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>