gmultipath failloop test: Add a checker for dtrace executes successfully or not

To provide a more informative error message.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 819961c5808b053c626648e202dec42a19ebe7a6)
(cherry picked from commit 38dac71d0a95ab2cf2cdfa1232cc556008b7ac94)
(cherry picked from commit b9b5a4dd590e1b30d92dc73b3d1f22d3303e7e41)