selftests: net: add config for openvswitch

The pmtu testing will require that the OVS module is installed,
so do that.

Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20240625172245.233874-8-aconole@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>