Skip sys.geom.class.multipath.failloop.failloop if dtrace fails to run

This test case depends on dtrace and sometimes gets affected if dtrace
has issues.  Make it report skipped instead of failure when dtrace fails
to run.

Sponsored by:	The FreeBSD Foundation

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

@@ -57,7 +57,7 @@ failloop_body()
 		-c "dd if=/dev/zero of=/dev/multipath/"$name" bs=4096 count=1" \
 		2>&1 | awk '/exited with status/ {print $NF}'`
 	if [ ! -f restore_count ]; then
-		atf_fail "dtrace didn't execute successfully"
+		atf_skip "dtrace didn't execute successfully"
 	fi
 	# The dd command should've failed ...
 	atf_check_equal 1 $dd_status