tests/geom: fix ATF test listing when geom class module is missing

geom_subr.sh serves both legacy TAP tests and ATF tests. It defaults to the
TAP path, so ATF tests must set ATF_TEST=true before sourcing it.

Sponsored by:	Netflix

tests/sys/geom/class/label/basic.sh

@@ -27,6 +27,7 @@
 #
 
 class=label
+ATF_TEST=true
 . $(atf_get_srcdir)/../geom_subr.sh
 
 atf_test_case create cleanup

tests/sys/geom/class/zoned/conf.sh

@@ -55,5 +55,6 @@ gzoned_test_cleanup()
 	true
 }
 
+ATF_TEST=true
 . `dirname $0`/../geom_subr.sh
 . `dirname $0`/../zoned_subr.sh