Add another G71 endless loop test

A stripped down version of the problem in
<LinuxCNC#2844>, code structure is
copied from 'g71-endless-loop'. The test is marked as skipped, since it
does not always terminates correctly.

tests/interp/g71-endless-loop2/expected

@@ -0,0 +1,31 @@
+ N..... USE_LENGTH_UNITS(CANON_UNITS_MM)
+ N..... SET_G5X_OFFSET(1, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000)
+ N..... SET_G92_OFFSET(0.0000, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000)
+ N..... SET_XY_ROTATION(0.0000)
+ N..... SET_FEED_REFERENCE(CANON_XYZ)
+ N..... ON_RESET()
+ N..... COMMENT("interpreter: feed mode set to units per revolution")
+ N..... SET_FEED_MODE(0, 1)
+ N..... SET_FEED_RATE(0.0000)
+ N..... SELECT_PLANE(CANON_PLANE_XZ)
+ N..... SET_SPINDLE_MODE(0 700.0000)
+ N..... SET_SPINDLE_SPEED(0, 250.0000)
+ N..... START_SPINDLE_CLOCKWISE(0)
+ N..... SET_FEED_RATE(0.2400)
+ N..... STRAIGHT_TRAVERSE(168.0000, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000)
+ N..... STRAIGHT_TRAVERSE(168.0000, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000)
+ N..... STRAIGHT_FEED(168.0000, 0.0000, -333.0000, 0.0000, 0.0000, 0.0000)
+ N..... STRAIGHT_FEED(170.0000, 0.0000, -333.0000, 0.0000, 0.0000, 0.0000)
+ N..... STRAIGHT_FEED(170.0000, 0.0000, -415.0000, 0.0000, 0.0000, 0.0000)
+ N..... STRAIGHT_TRAVERSE(300.0000, 0.0000, 10.0000, 0.0000, 0.0000, 0.0000)
+ N..... SET_G5X_OFFSET(1, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000)
+ N..... SET_XY_ROTATION(0.0000)
+ N..... SELECT_PLANE(CANON_PLANE_XY)
+ N..... SET_FEED_MODE(0, 0)
+ N..... SET_FEED_RATE(0.0000)
+ N..... STOP_SPINDLE_TURNING(0)
+ N..... SET_SPINDLE_MODE(0 0.0000)
+ N..... PALLET_SHUTTLE()
+ N..... PROGRAM_END()
+ N..... ON_RESET()
+ N..... ON_RESET()

tests/interp/g71-endless-loop2/g71-endless-loop2.ngc

@@ -0,0 +1,12 @@
+G18G95
+G96 M3 S250 D700
+
+O2 SUB
+Z-333
+G1 X170
+Z-415
+O2 ENDSUB
+G71 Q2 X168 Z0 F.24
+
+G0 X300 Z10
+M30

tests/interp/g71-endless-loop2/test.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+#
+# Demonstrate endless loop when an additinal segment is added in the G71 call.
+# This is a subset of a file that was supplied in
+# https://github.com/LinuxCNC/linuxcnc/issues/2844 and which worked up to and
+# including 2.9.1~pre1. It entered an endless loop after
+# https://github.com/LinuxCNC/linuxcnc/pull/2677
+
+rs274 -g g71-endless-loop2.ngc | awk '{$1=""; print}' > result &
+pid=$!
+
+# Give it 5 seconds to complete
+count=5
+while [ 0 -lt $count ] && kill -0 $pid > /dev/null 2>&1 ; do
+    sleep 1
+    count=$(($count - 1))
+done
+
+if kill -0 $pid > /dev/null 2>&1; then
+    kill -9 $pid
+    echo "error: g71-endless-loop2.ngc program seem to be stuck, killing"
+    exit 1
+fi
+
+exit 0