gh-97983: Revert "Lay the foundation for further work in asyncio.test…

…_streams: port server cases to IsolatedAsyncioTestCase" (#98015)

This PR reverts gh-93369 and gh-97896 because they've made asyncio tests unstable. After these PRs were merged, random GitHub action jobs of random commits started to fail unrelated tests and test framework methods.

The reverting is necessary because such shrapnel failures are a symptom of some underlying bug that must be found and fixed first.

I had a hope that it's a server overload because we already have extremely rare disc access errors. However, one and a half day passed, and the failures continue to emerge both in PRs and commits.

Affected issue: gh-93357.
First reported in #97940 (comment).

* Revert "gh-93357: Port test cases to IsolatedAsyncioTestCase, part 2 (#97896)"

This reverts commit 09aea94.

* Revert "gh-93357: Start porting asyncio server test cases to IsolatedAsyncioTestCase (#93369)"

This reverts commit ce8fc18.