[flang][NFC] Add namelist lowering test

This patch adds a lowering test for the namelist.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D122318



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Peter Steinfeld <psteinfeld@nvidia.com>
Co-authored-by: V Donaldson <vdonaldson@nvidia.com>