[flang] Improve failure message from test_modfile.sh

When test_modfile.sh fails a test, show the path to the source file
so it is easier to find the problems when grepping the log.

Original-commit: flang-compiler/f18@4a0d0d19a0010b9342425a9b7f1db3ce4dada661
Reviewed-on: https://github.com/flang-compiler/f18/pull/630
Tree-same-pre-rewrite: false