[gnulib-tool-py] --megatest minor bug

[Stefano Lattarini]

Hello gnulibers.

I've found an incompatibility with between mainline gnulib-tool and
gnulib-tool.py.  With mainline gnulib-tool, the following:

    ./gnulib-tool --megatest --with-tests y0 yn

works as expected, testingthe y0 and yn modules first separately and
then in combination.  But with gnulib-tool.py:

    $ python ./gnulib-tool.py --megatest --with-tests y0 yn
    usage: gnulib-tool.py --help
    ./gnulib-tool.py: error: unrecognized arguments: y0 yn

I've not investigated further.

Regards,
  Stefano