Fix `jest/valid-expect` lints throughout the codebase

[weeman1337]

This is a follow-up to matrix-org/matrix-react-sdk#10261, sister issue tracking the same thing in the matrix-js-sdk repo,

There are too many flawed tests which violate jest/valid-expect and don't pass by simply adding await to the assertions (they require more http mocking, etc). We should fix up these scenarios as they are probably masking some failure cases.

The rule is currently disabled in .eslintrc.js