py/parse: Remove unnecessary check in const folding for ** operator.
py/parse: Remove unnecessary check in const folding for ** operator.

In this part of the code there is no way to get the ** operator, so no need
to check for it.

This commit also adds tests for this, and other related, invalid const
operations.

---

Updated for Pycopy.

Change-Id: If5d3295f8a5741172badfdad444405d137cffdf6
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython