[flang] Fix a test (use %s instead of $s)

With the typo ($S instead of %s), the driver was expecting
input from stdin. In such cases, it prints:
```
Enter Fortran source
Use EOF character (^D) to end file
```
This was piped to FileCheck. Together with the available `CHECK-NOT`
statement, this was sufficient for the test to pass (incorrectly).

This patch makes sure that the provided input file is used instead of
stdin.

Differential Revision: https://reviews.llvm.org/D100301