nixos/test-driver: deduplicate VLANs

Naively deduplicate VLANs in the python driver for NixOS tests. The
current implementation accidentally works, since the VLan class mutates
the environment. On construction it sets QEMU_VDE_SOCKET_${id} and this
environment variable gets overwritten once a second VLAN with the same
id is constructed. Because the NIC flags passed to qemu just use the
QEMU_VDE_SOCKET_${id} environment variable, this implicitly chooses a
single vde_switch process for each VLAN.

However, this leads to unusable vde_switch processes being spawned in
each test run and as a side effect makes it impossible to access the
correct VLan objects in the interactive test driver. It also makes it
remarkably hard to understand why the current implementation ever
worked.

nixos/lib/test-driver/test_driver/driver.py

@@ -55,6 +55,7 @@ class Driver:
         tmp_dir = get_tmp_dir()
 
         with rootlog.nested("start all VLans"):
+            vlans = list(set(vlans))
             self.vlans = [VLan(nr, tmp_dir) for nr in vlans]
 
         def cmd(scripts: List[str]) -> Iterator[NixStartScript]: