[llvm] [Flang-RT][test] Add flang-rt-mod dependency (PR #201297)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

I noticed that the `Performing configure step for 'runtimes'` output looks like from an incremental build that uses the CMakeCache.txt from a previous run. In particular, this is missing:
```
-- The Fortran compiler identification is LLVMFlang 23.0.0
-- Detecting Fortran compiler ABI info
-- Detecting Fortran compiler ABI info - done
-- Check for working Fortran compiler: /home/meinersbur/src/llvm/flangrt/cache2/./bin/flang - skipped
-- Fortran support enabled using just-built Flang-RT builtin modules
```
How can that be? I see a `clean build` step.

https://github.com/llvm/llvm-project/pull/201297