[llvm] [flang-rt] Update `test_flang` in CI to use `flang-rt` as one of the projects. (PR #135260)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

The PR may have dependency on PR #134362 as it will need the driver change to get the full path to `libflang_rt.runtime.a`. 

https://github.com/llvm/llvm-project/pull/135260