This test verifies that M62-M68 can be remapped through both python
and NGC, and that calling the original M-codes is supported in the
remap subroutine.

It also serves as an example of how to do this, since esp. the M66
python function is not trivial and must be a generator yielding
`INTERP_EXECUTE_FINISH`.

The recursive call to the original functions needs supporting code for
mode 5 M-codes in `interp_convert.cc` `convert_m()`, and for M62-M68
in `rs274ngc_pre.cc`, `find_remappings()`.

The peculiar M66 needs special handling in the interpreter for
`input_flag`, and to ensure that its result (in param `#5399`) is
available within the remap, `Interp::pycall()` runs `read_inputs()`
before returning control back to the python generator.
