The test assumed that `ggatel create` created a device on completion, but that's
incorrect. This squashes the race by waiting for the device to appear, as
`ggatel create` daemonizes before issuing an ioctl to geom_gate(4) if not called
with `-v`.
Discussed with: asomers
MFC after: 1 week
PR: 204616
Sponsored by: Dell EMC Isilon