tests: net: Fix tests so they can be run in real hw

Both the vlan and checksum_offload tests expect to be run in
controlled environment where the test application is providing
ethernet connectivity. If the board under test has also ethernet
support, then internal checks will fail in the test application.
Because of this, disable on board ethernet so that the tests will
pass.

Fixes #8172

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