nixos/tests: fix hostname under network-online dep fix [BROKEN]

Changed files
+1
nixos
tests
+1
nixos/tests/hostname.nix
···
machine = ${hostName}
machine.wait_for_unit("network-online.target")
# Test if NixOS computes the correct FQDN (either a FQDN or an error/null):
···
machine = ${hostName}
+
machine.systemctl("start network-online.target")
machine.wait_for_unit("network-online.target")
# Test if NixOS computes the correct FQDN (either a FQDN or an error/null):