- Ensure we are unserialising the function coverage

- This issue presents when running the test suite for the first time
  (e.g. in CI) since there is no `tests/_files/tmp` directory created.
- In the one test case which requires the cleanup of these files because
  it creates them it does not fail because the directory is created.
- The other test cases don't need this tear down, but they fail when it
  tries to run as this directory has not yet been created.
- In future we'll probably remove both the tear down and the test case
  that needs it as this is probably not an appropriately direct way to
  test this.