Use pytest fixtures: https://pytest.org/latest/fixture.html

There is already a `fakefile` fixture in [conftest.py](https://github.com/CenterForOpenScience/modular-file-renderer/blob/dev/conftest.py). This means that it can be used in *any* test within the package. 

To use it, just add it as an argument to a test function

```python
def test_image_handler_does_something(fakefile):
    fakefile.name = 'myfile.jpg'
    # assert something