tests: net: Run networking tests only for selected platforms

Run networking tests for native_posix, qemu_x86 and qemu_cortex_m3
platforms only as the tests are generic enough so no need to run
them in real physical device.

Fixes #9623 #9614 #9622 #9621 #9618

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>