7ad06473b6 split out `LOCAL_AVATARS_DIR` and `LOCAL_FILES_DIR` as
derived values from `LOCAL_UPLOADS_DIR`. However, this means that all
places which set `LOCAL_UPLOADS_DIR` need to potentially propagate
that change into the derived values if they come *after*
`computed_settings.py` is applied. It did this successfully in
`zerver/lib/test_runner.py` and the `use_s3_backend` decorator, but
did not adjust the late-set `zproject/test_extra_settings.py`.
This causes tests to share a single common set of avatars and
attachments directories. In puppeteer tests, this leads to assertion
failures checking `assert_is_local_storage_path`; in backend tests,
this leads to races when checking the contents of the local storage
directory when run in parallel mode.
Set `LOCAL_AVATARS_DIR` and `LOCAL_FILES_DIR` based on
`LOCAL_UPLOADS_DIR` when it is pulled from the environment during
testing.
dcd8947c1b