[Refactor] [python] [testing] Rename vague backend test fixtures (e.g. `do_something`) to descriptive names

[khalatepradnya]

Several Python backend tests use a session-scoped, autouse fixture named do_something to set the CUDA-Q target and perform teardown. The name is not descriptive.

Replace these instances with something meaningful like set_up_target or reset_target_and_clear_registries.

[Ahmedadel87]

I would like to try

[Ahmedadel87]

I did the change renamed do_something to set_up_target
PR: #4087