Temporarily skip sys.geom.class.multipath.failloop.failloop in CI

This test case uses `dtrace -c` but it has some issues at the moment

While here, add a checker for dtrace executes successfully or not to provide
a more informative error message.

PR:             258763
Sponsored by:   The FreeBSD Foundation

tests/sys/geom/class/multipath/failloop.sh

@@ -36,6 +36,10 @@ failloop_head()
 }
 failloop_body()
 {
+	if [ "$(atf_config_get ci false)" = "true" ]; then
+		atf_skip "https://bugs.freebsd.org/258763"
+	fi
+
 	sysctl -n kern.geom.notaste > kern.geom.notaste.txt
 	load_gnop
 	load_gmultipath
@@ -59,6 +63,9 @@ failloop_body()
 	# The dd command should've failed ...
 	atf_check_equal 1 $dd_status
 	# and triggered 1 or 2 path restores
+	if [ ! -f restore_count ]; then
+		atf_fail "dtrace didn't execute succfully"
+	fi
 	if [ `cat restore_count` -gt 2 ]; then
 		atf_fail "gmultipath restored paths too many times"
 	fi