Load geom_gate(4) if necessary before running tests; skip if it can't be loaded

The test code prior to r311893 loaded geom_gate at test start if necessary and
skipped the tests if it couldn't be loaded.

The ATF-ifcation of this test done in r311893 unfortunately dropped this
functionality.

This change restores the geom_gate module load and skips the test(s) if unavailable
in an ATF-like way.

MFC after:	1 month
PR:		220164
Reported by:	gjb

tests/sys/geom/class/gate/ggate_test.sh

@@ -16,6 +16,8 @@ ggated_head()
 
 ggated_body()
 {
+	load_ggate
+
 	us=$(alloc_ggate_dev)
 	work=$(alloc_md)
 	src=$(alloc_md)
@@ -57,6 +59,8 @@ ggatel_file_head()
 
 ggatel_file_body()
 {
+	load_ggate
+
 	us=$(alloc_ggate_dev)
 
 	echo src work >> ${PLAINFILES}
@@ -91,6 +95,8 @@ ggatel_md_head()
 
 ggatel_md_body()
 {
+	load_ggate
+
 	us=$(alloc_ggate_dev)
 	work=$(alloc_md)
 	src=$(alloc_md)
@@ -195,6 +201,19 @@ common_cleanup()
 	true
 }
 
+load_ggate()
+{
+	local class=gate
+
+	# If the geom class isn't already loaded, try loading it.
+	if ! kldstat -q -m g_${class}; then
+		if ! geom ${class} load; then
+			atf_skip "could not load module for geom class=${class}"
+			exit 0
+		fi
+	fi
+}
+
 # Bug 204616: ggatel(8) creates /dev/ggate* asynchronously if `ggatel create`
 #             isn't called with `-v`.
 wait_for_ggate_device()