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