[MLIR] Rename test/python/dialects/math.py -> math_dialect.py

Rename test/python/dialects/math.py -> math_dialect.py to avoid a
collision with a Python standard package of the same name. These test
scripts are run by path and are not part of a package. Python apparently
implicitly adds the containing directory to its PYTHONPATH. As such,
test scripts with common names run the risk of conflicting with global
names and resolution of an import for the latter happens to the former.

Differential Revision: https://reviews.llvm.org/D114568