openharmony-standard-image-tests: Use tun/tap networking

As we by now require working tun/tap support in CI runner to allow running ACTS,
there is no need for using slirp networking.

And who knows, maybe the randomly failing ssh ptest could be related to the QEMU
slirp implementation.

Signed-off-by: Esben Haabendal <esben@geanix.com>