[llvm] [flang] Enable Driver/exec.f90 test (NFC) (PR #170544)

[Kelvin Li via llvm-commits]

kkwli wrote:

> Linking an executable requires libflang_rt.runtime.a/so, so the test would fail if it is not present. Did I forget to define "flang-rt" in lit.site.py if LLVM_ENABLE_RUNTIMES=flang-rt?

My understanding is that this test is in flang-rt and is part of `check-flang-rt`. I assume that the `libflang_rt.runtime.a/so` would have been successfully built before doing `check-flang-rt`. 

I don't see `config.available_features.add("flang-rt")` in `flang-rt/test/lit.cfg.py`.

https://github.com/llvm/llvm-project/pull/170544