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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

>  so it uses bootstrap build instead of part of the flang build

So adding `flang-rt` to `LLVM_ENABLE_PROJECTS` will cause it to use a just-built clang/flang (although a cursory glance seems to show there is no Fortran code in `flang-rt`?) for compilation? That is completely different from how all the other runtimes work (they use the host compiler for `LLVM_ENABLE_PROJECTS` and will use a just built compiler when specified in `LLVM_ENABLE_RUNTIMES`).

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