Changes

[clang-format] NFC keep the code clang-formatted

[flang][driver] Refactor unit tests for frontend actions (nfc)

These patch implements a few non-functional-changes:
  * switch to using test fixtures for better code sharing
  * rename some variables (e.g. to communicate their purpose a bit better)
This patch doesn't change _what_ is being tested.

Differential Revision: https://reviews.llvm.org/D93544