[th/prepare_patched_netdevsim-race] fix race with @prepare_patched_netdevsim to wait for device being ready

Otherwise, there is a race with test @ethtool_features_coal:

First we create the netdevsim interfaces, but they are not yet ready (have carrier) by the time the test issues nmcli connection up.

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Example:

https://jenkins-networkmanager.apps.ocp.ci.centos.org/job/NetworkManager-code-mr/355/artifact/FAIL-report_NetworkManager-ci_Test0533_ethtool_features_coal.html