motion: Fix motion::triple_move step count limit

Do not assume any step can be merged while moving three axes at the same
time.

Use the worst case scenario involving indepedent parameters for each
axis.

tests/unit/modules/motion/test_motion.cpp

@@ -216,7 +216,7 @@ TEST_CASE("motion::triple_move", "[motion]") {
     motion.PlanMoveTo(Pulley, p, 1);
 
     // perform the move with a maximum step limit
-    REQUIRE(stepUntilDone(std::max({ i, s, p }) + 1) != -1);
+    REQUIRE(stepUntilDone(i + s + p) != -1);
 
     // check queue status
     REQUIRE(motion.QueueEmpty());