make all fixtures to be function scoped

We should provide function scoped by default for correctness, and we can offer fixtures with a different scope if needed.

Ideally, it'd look something like this:
```python
def s3_server_impl():
   yield MockedS3Server()

@pytest.fixture():
def s3_server():
   yield s3_server_impl()

@pytest.fixture(scope="session"):
def session_s3_server():
   yield s3_server_impl()

...
```