Add a testcase for look-ahead (smooth curves).
gcode-process.c: fix HEATER_EXTRUDER reference.

A rare combination of factors makes this code assume HEATER_EXTRUDER
always exists, when that is not necessarily so.  Add the normal
guard around it.
Remove stale reference to endstops_stop

This was accidentally left out of the change at 13ec2d75.
Add another look-ahead testcase.