Add mock s3 test

[valeriupredoi]

Description

We mock an entire S3 fsspec/s3fs-like File System, pop a working HTTP client on top of it, and add a real POSIX file to it, then access it as if it was a real file but in an S3 s3fs-like FS. Wild Weasel YGBSM stuff 😁

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• All tests pass