Wait for /dev/ggate* to appear after calling ggatel create in :ggatel_{file,md}

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