Don't assume cwd is root dir in relative paths in `conftest.py`

There's lots of filepaths in `conftest.py` which are relative to the root dir, and so if tests are executed in a non-root dir then these paths will be incorrect and cause test failures: https://github.com/DiamondLightSource/httomo/blob/833936aaa1049feac5c55a2195e99454dee85fcc/tests/conftest.py#L195-L294

Replacement with something like `Path(__file__) / "path/relative/to/conftest/file"` would resolve this