1. Replace for/retry loops with "wait_for_ggate_device" calls

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 to create /dev/ggate* devices.

Switch from retry loops to wait_for_ggate_device function calls 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

2. Use atf_check consistently with all dd calls to ensure that data output is expected.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

for i in $(seq 10); do \
sudo kyua test -k /usr/tests/sys/geom/class/gate/Kyuafile \

	    ggate_test:ggated || break; \

done

->

...
ggate_test:ggated -> passed [27.406s]

Results file id is usr_tests_sys_geom_class_gate.20170201-164729-582240
Results saved to /root/.kyua/store/results.usr_tests_sys_geom_class_gate.20170201-164729-582240.db

1/1 passed (0 failed)
$ echo $? $i
0 10
$