test: fix OVS teardown race with OVSDB

OVS dup-name fixture teardowns used `nmcli connection del`, which is
async with respect to OVSDB cleanup. Replace with `libnmstate.apply`
using InterfaceState.ABSENT, which waits for full state convergence.

Signed-off-by: Vladimír Beneš <vbenes@redhat.com>