MFC r311893, r313008, r313081

r311893:
ATFify the geom gate tests.

This ensures their cleanup routines will be run even if they should timeout.

tests/sys/geom/class/gate/ggate_test.sh
tests/sys/geom/class/gate/Makefile
Add an ATF test with three testcases, one for each TAP test. Use
ATF-style cleanup functions, and convert sleeps to polling loops.

ObsoleteFiles.inc
tests/sys/geom/class/gate/conf.sh
tests/sys/geom/class/gate/1_test.sh
tests/sys/geom/class/gate/2_test.sh
tests/sys/geom/class/gate/3_test.sh
Delete TAP test files

Reviewed by: ngie
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D8891

r313008:
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

r313081:
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