selftests: gpio: gpio-aggregator: add a test case for _sysfs prefix reservation

The kernel doc for gpio-aggregator configfs interface, which was recently
added, states that users should not be able to create an aggregator with a
name prefixed by "_sysfs" via configfs. However, it was found that this
guard does not function as expected (thanks to Dan Carpenter for
identifying and fixing the issue).

Add a test case to verify the guard.

Signed-off-by: Koichiro Den <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

tools/testing/selftests/gpio/gpio-aggregator.sh

···
 agg_remove_line   agg0 line0
 agg_remove_chip   agg0
 
+echo "1.1.6. Can't instantiate a chip with _sysfs prefix"
+mkdir "$CONFIGFS_AGG_DIR/_sysfs" 2> /dev/null && fail "chip _sysfs unexpectedly created"
+mkdir "$CONFIGFS_AGG_DIR/_sysfs.foo" 2> /dev/null && fail "chip _sysfs.foo unexpectedly created"
+
 echo "1.2. Creation/deletion via sysfs"
 
 echo "1.2.1. Minimum creation/deletion"