Add testcase for nullmoves.

Moves which have no movement intention, e.g. pure feedrate changes,
and moves too small to cause a single step, are a bit tricky to
handle with lookahead. Essentially, they should be joined with the
next movement, without queueing them up.

testcases/nullmoves.gcode

@@ -0,0 +1,16 @@
+(run this with STEPS_PER_M_X < 500000)
+(for example, the typical 80000)
+G21
+G90
+G1 X5 F400
+G1 X0
+G1 F200
+G1 X10
+G1 F400
+G1 X20
+G1 X20.001
+G1 X20.002
+G1 X20.003
+G1 X25
+G1 X0
+M2

testcases/straight-speeds.gcode

@@ -13,4 +13,4 @@ G1 X10 Y10 F200
 G1 X20 Y20 F400
 G1 X100 Y100 F1500
 G1 X0 Y0
-M2
+M2