Update zarr ingestion test to ensure unique faker pystr values

I'm not sure of the randomness of `pystr`, but it's generally a good idea to include the iterated value in the generated path to prevent collisions (`path=f'foo/baz/{i}_{faker.pystr()}'`). Especially with 1005 entries, you're likely to hit one eventually.

_Originally posted by @dchiquito in https://github.com/dandi/dandi-archive/pull/914#discussion_r816910745_

Locate the zarr ingestion test that creates 1005 entries and inspect how it uses faker.pystr() to build generated paths. Run the test, then verify that the paths incorporate the iterated value and remain unique across all entries.

Written by the indexing model from the issue text.