[flang] Fix bug in test_modfile.sh

The expected mod files specified in the test do not necessarily
appear in sorted order, and the desired order depends on what the
`comm` command expects. So sort them to be sure it is right.

Original-commit: flang-compiler/f18@730d0d0562dd0a4eabfde020fef9fe371d401ff5
Reviewed-on: https://github.com/flang-compiler/f18/pull/309