[flang] Add a .mod file test with subprograms

The .mod file just contains the API. Test that prefixes and suffixes
on the declaration get correctly persisted.

Original-commit: flang-compiler/f18@eb2a6deb4fe12bae83fc3cb2366d056cf50cc9a5
Reviewed-on: https://github.com/flang-compiler/f18/pull/126
Tree-same-pre-rewrite: false