[3.12] GH-120754: Add more tests around seek + readall (GH-122103) by miss-islington · Pull Request #122216 · python/cpython

In the process of speeding up readall, A number of related tests
(ex. large file tests in test_zipfile) found problems with the
change I was making. This adds I/O tests to specifically test these
cases to help ensure they don't regress and hopefully make debugging
easier.

This is part of the improvements from
https://github.com/python/cpython/pull/121593GH-issuecomment-2222261986
(cherry picked from commit 9eb7341)

Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>