This PR formats all the `*.py` files found under the `tests/integration`
folder. It also reorders the imports and cleans up a bunch of unused
imports.
The formatting also takes care of other things like wrapping lines and
fixing spaces and indents such that the tests look more readable.
* Fixed `merge_with_ttl_timeout` logic.
* Separate TTL-merge timers for each partition
Co-authored-by: Vladimir Chebotarev <vladimir.chebotarev@gmail.com>
