Follow up to r312118

State that execve failed instead of just printing out the program name
and strerror(errno) via err(3).

MFC after:	3 days
X-MFC with:	r312118
Sponsored by:	Dell EMC Isilon

tests/sys/kern/execve/execve_helper.c

@@ -50,5 +50,5 @@ main(int argc, char **argv)
 	}
 
 	execve(argv[1], &argv[1], NULL);
-	err(1, "%s", "");
+	err(1, "execve failed");
 }