On one of the Jenkins agents for macOS Catalina, the `Face/TestMulticastEthernetTransport/NetifStateChange` consistently fails when it attempts to use the `awdl0` (Apple Wireless Direct Link) interface (which is administratively up, but is not RUNNING). However, I have confirmed that it still fails on macOS when this test case attempts to use a standard Ethernet interface that is also administratively up, but not RUNNING. Davide and I have determined that this issue is likely caused by a combination of the following two issues: * EthernetTransport is not initially set to `TransportState::DOWN` DOWN when the underlying interface is initially not `InterfaceState::RUNNING`. RUNNING. * EthernetFixture uses any interface that is administratively up not DOWN, but not necessarily `InterfaceState::RUNNING`, while the test case in question assumes that the transport is initially `TransportState::UP` RUNNING (not guaranteed once the previous point is fixed). guaranteed).