Make it harder for pytests to use real AWS resources

[timsnyder-siv]

Yeah, me too. Makes me think even more that we should just do the stuff in that fixture by default in conftest.py and swap the requirement to be so that people who want to really get out to AWS during pytest have to do work to undo the fake credentials.

Originally posted by @timsnyder-siv in https://github.com/firesim/firesim/pull/1155#discussion_r938166784

[timsnyder-siv]

The tests currently have to add a aws_test_credentials fixture to prevent connecting to AWS. I think those should be applied universally in testconf.py and anyone wanting to really connect to AWS from inside a pytest should need to remove them.