Cannot assign requested address on first test run after creating bridge

[alfreb]

The first test run after creating the network bridge fails every time. Subsequent runs succeed.

Repro:

1. If you already have bridge43, delete it: sudo ip link delete bridge43
2. create the bridge with e.g. ~/vmrunner/vmrunner/bin/create_bridge.sh
3. Now run the test: nix-shell --pure $CCACHE_FLAG --argstr unikernel ./test/net/integration/udp --run ./test.py

<vm> [ UDP test service ] Listening on port 4242
<Test.py> Performing UDP6 tests
[ WARNING ] Exception raised in event callback:
Traceback (most recent call last):
  File "/nix/store/aw5c2j3q34fd8fnm7s2qdwipxddi40ll-python3-3.11.9-env/lib/python3.11/site-packages/vmrunner/vmrunner.py", line 1081, in trigger_event
    res = func(line)
          ^^^^^^^^^^
  File "/run/user/1001/tmp.6FGGRVPn6n/./test.py", line 75, in UDP6_test
    sock.sendto(data, addr)
OSError: [Errno 99] Cannot assign requested address