fix(test): add __init__.py files

We were seeing "import mismatch error" due to files in functional and
performance having the same name. According to [1], a fix is to just
have __init__.py files.

[1]: https://stackoverflow.com/questions/53918088/import-file-mismatch-in-pytest

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat