Same guard the ethernet-switch scenario already carries: NetworkManager
can release an NM-managed TAP from its bridge shortly after
(re-)enslavement (docs/bugs/networkmanager-tap-release.md). Applied to
the Dynamips, QEMU, IOU, IOL-runner and switch-cascade scenarios after
their nodes have booted (no-op where nmcli/NetworkManager is absent).
This keeps those tests deterministic on un-declared hosts.