Update pytest fixture for async functions

[Tianqi Xiao]

Starting from `pytest-asyncio`'s most recent release (version 0.19.0), strict mode is enforced by default [1], which requires the usage of `@pytest.mark.asyncio` and `@pytest_asyncio.fixture` [2]. Without these markers, tests and fixtures are not considered as asyncio-driven, resulting functional tests failures.

For charms using pytest framework, we need to update their functional tests by replacing the current used `@pytest.fixture` with `@pytest_asyncio.fixture` in test files.

[1]: https://github.com/pytest-dev/pytest-asyncio/issues/390
[2]: https://github.com/pytest-dev/pytest-asyncio#strict-mode