Add a testcase for moves shorter than the acceleration ramps.

testcases/short-moves.gcode

@@ -0,0 +1,27 @@
+G21
+G90
+X3 F300
+X0
+X1
+X2
+X3
+X3.5
+X4
+X4.4
+X4.8
+X5.1
+X5.4
+X5.6
+X5.8
+X6.0
+X6.1
+X6.2
+X6.3
+X6.4
+X6.5
+M2
+; For 50 mm/s2. Move 1 & 2 for lookahead-warmup.
+; Move 3 to 5 are long enough to reach target speed.
+; Move 6 to 11 allow full rampup or rampdown, but not both.
+; Move 12 to 19 are even shorter than one ramp.
+; All should run in a single ramp.