[flang] Add tests for .mod file writing

The source files contain the expected contents of generated .mod files.
`test_modfile.sh` compiles the source file and verifies that the correct
.mod files are generated.

Original-commit: flang-compiler/f18@a23f53c1a7d6f1dba157c1bc87ebc046d3b818e6
Reviewed-on: https://github.com/flang-compiler/f18/pull/126
Tree-same-pre-rewrite: false