Skip two flaky tests and tweak more test timeouts

[Eric-Arellano]

Skip some exception sink integration tests on macOS

These shards have chronically flaked by hanging since at least July #8127. They are our most egregious Python flake.

We will still test these four tests on Linux and only skip them on macOS, as this is a macOS specific
issue.

Skip remote::tests::dropped_request_cancels

This seems to flake roughly 40% of the time #8405. It is our most egregious Rust flake.

Tweak some other tests

We mark some other tests as flaky and bump their timeouts as relevant to hopefully stabilize CI further.

[Eric-Arellano]

Incredible: I think this is the first time I've had a CI run be green on the first try in over 2 months 🎉

(Although, we shouldn't be too excited. The way we got this was through skips and retries, rather than fixing the underlying issues.)