Create an s3/gate subscope in the s3 test set

[vvarg229]

There is a test set for compatibility with S3 API. This set has nothing to do with the specifics of the fact that S3 in NeoFS is served through a gate, not directly from the cluster. So if we are testing only S3 API, there is no point in writing about the gate. But if we test the gate as a standalone application (process) with its custom specifics (for example, reaction to a particular OS signal), then it is worth creating a set of s3/gate tests in the s3 test suite.

it's a bit strange to me to see test_s3_gate.py file in s3_gate besides seeing S3 API tests in it. Is this file intended for NeoFS S3 Gateway specifics not covered by the protocol?

i've left some questions and suggestions from my own, if u'll find any of them useful, feel free to postpone them via issues, i understand that this PR is about migration, not enhancements

i don't plan to look closely at the other tests (at least for now), hope @evgeniiz321 @roman-khimov will help

Originally posted by @cthulhu-rider in #699 (review)