[NFC][flang][do concurent] Add saxpy offload tests for OpenMP mapping (#155993)
Adds end-to-end tests for `do concurrent` offloading to the device.
PR stack:
- https://github.com/llvm/llvm-project/pull/155754
- https://github.com/llvm/llvm-project/pull/155987
- https://github.com/llvm/llvm-project/pull/155992
- https://github.com/llvm/llvm-project/pull/155993 ◀️
- https://github.com/llvm/llvm-project/pull/157638
- https://github.com/llvm/llvm-project/pull/156610
- https://github.com/llvm/llvm-project/pull/156837