[flang][driver] Refactor one unit-test case to use fixtures (nfc)

Move the unit test from InputOutputTest.cpp to FrontendActionTest.cpp
and re-implement it in terms of the FrontendActionTest fixture. This is
just a small code clean-up and a continuation of:
  * https://reviews.llvm.org/D93544

Moving forward, we should try be implementing all unit-test cases for
Flang's frontend actions in terms of FrontendActionTest.

Reviewed By: sameeranjoshi

Differential Revision: https://reviews.llvm.org/D94922