TST: dask tests are extremely flaky

[crusaderky]

Dask is running on 5 cases per test, causing it to be extremely flaky.
This PR brings it back in line with other libraries to 200.
This PR tweaks xfails and skips after running locally on 2000 examples and adds some commentary.

[ev-br]

All other test runs are under five minutes, and even with max-examples 5, dask tests are the slowest. I'd rather strongly prefer all tests here to run under a few minutes.

Even the default 200 examples is more of a smoke-testing really. When looking for regressions or working on a new feature, I regularly run with --max_examples=10_000 locally. Not on dask, of course, that would never complete.

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The state of the CI currently is not great indeed. One possibility is to have a nightly job which runs many examples.
One other idea is to export arrays in "unvectorized" tests to dlpack or something: data-apis/array-api-tests#197 (comment).

[crusaderky]

This is very frustrating. I've reverted the number of examples back to 5 and ran locally with 2000 examples. The updated xfails and skips files reflect that.

[ev-br]

Thanks @crusaderky !
It is frustrating, yes. Suggestions for improvements are most welcome indeed