[Flang] Port test_folding.sh to Python

To enable Flang testing on Windows,
shells scripts have to be ported to Python.
The following changes have been made:
Ported `test_folding.sh` to Python;
Additional changes to the tests themselves
to use the new script.

LIBPGMATH support for testing
not available at this point.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D108217