Replace for/retry loops with "wait_for_ggate_device" calls and check
results of commands

As noted in r313008, the underlying issue was that geom_gate device
creation wasn't created at ggatel command completion, but some short
time after. ggatec(8) employs similar logic when creating geom_gate(4)
devices.

Switch from retry loops (after the ggatec/dd write calls) to
wait_for_ggate_device function calls after calling ggatec(8) instead
to detect the presence of the /dev/ggate* device, as this function is
sufficient for determining whether or not the character device is ready
for testing

While here, use atf_check consistently with all dd calls to ensure that
data output is as expected.

MFC after: 1 week
Reviewed by: asomers
Differential Revision: D9409
Sponsored by: Dell EMC Isilon