feat: add expect.poll utility

[sheremet-va]

Description

When rewriting UI tests, I found that we don't have an assertion resistant to flaky values. This should make it easier to check if a flaky condition is working correctly. I also expect to have this functionality built-in in future browser-only assertions.

TODO:

• Docs
• Tests

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• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
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Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

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[AriPerkkio]

How is this different from

await vi.waitFor(() => {
  expect(..).toBe(...)
});

[sheremet-va]

How is this different from

await vi.waitFor(() => {
  expect(..).toBe(...)
});

It's more compact and easier to understand the idea behind it. Having several vi.waitFor just doesn't look as clean:

await createElement()

await vi.waitFor(() => {
  expect(getElement()).toBeInTheDocument()
})

await clickOnElement()

await vi.waitFor(() => {
  expect(getResult()).toEqual({ pass: true })
})

await createElement()

await expect.poll(() => getElement()).toBeInTheDocument()

await clickOnElement()

await expect.poll(() => getResult()).toEqual({ pass: true })

Playwright also has await expect(() => {}).toPass() API which looks more like vi.waitFor