Test GET of a single part from a multipart object

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we don't have tests for things S3 gateway provides. nspcc-dev/neofs-s3-gw#1260 solved nspcc-dev/neofs-s3-gw#1224 and we can now get a specific part of multipart upload after upload completion.

Describe the solution you'd like

Very likely there are some tests for it in https://github.com/nspcc-dev/s3-tests, but they're currently disabled. Check/enable them. If there are no such tests, create them here as generic S3 ones.