Consider that we stop using `container` in favor of `document.body` in tests

[julienw]

In #3305 I disabled the new eslint test that checks when we use container. We do use container in tests for these cases:

1. snapshot tests
2. use of querySelector and friends

Advantages of using document.body:

• this is standard DOM
• we don't need to forward container from the render result to the test code
• there's no difference with fragments or portals when doing queries
• the queries getByText and friends are using document.body already.

Drawbacks of using document.body:

• for Flow document.body can be null, so this can be painful to use ensureExists or $FlowExpectError eveywhere.
• more nested output in snapshots

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