tests/network: avoid race conditions with NetworkManager
tests/network: avoid race conditions with NetworkManager

The "netvm" in the tests is configured manually, using "ip" command, and
by manually writing to /etc/resolv.conf. Do not let NetworkManager
interfere - simply disable it.

QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin