Add missing dependencies on the flang test target

Without this, the LLVM utilities (FileCheck) aren't built when running
`ninja check-flang` and it fails with:

llvm-project/llvm/utils/lit/lit/llvm/subst.py:134: fatal: Did not find FileCheck in...

Also the modules aren't built without depending on `module_files`, which
makes multiple tests failing.

Differential Revision: https://reviews.llvm.org/D78036