[clang] [NFC][clang][Driver] Add tests for --driver-mode=flang (PR #207658)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu commented:

Apologies for being picky. Just a few more things. Thank you very much for your patience.

https://github.com/llvm/llvm-project/pull/207658