Include a valid nipoppy imaging status file in our test data

[surchs]

Nipoppy has a schema for imaging bagel/status files. The schema is in pydantic and we might eventually use it directly to validate inputs (#352). Until then we want to make sure that we stay current with the schema. To do so we will

• grab example imaging status file from a nipoppy maintained repo (e.g. catalog) and
• use it as a test target.
• Keep this example in sync with upstream nipoppy (e.g. submodule) so
• tests start breaking when we go out of sync

Originally posted by @surchs in #352