add `toHaveCount` assertion for playwright locator

[jnizet]

Clear and concise description of the problem

To test that a locator matches a given number of elements, in polling mode, we currently have to do (unless I'm mistaken)

await expect.poll(() => locator.elements().length).toBe(3);

I think that kind of assertion is quite frequent, and Playwright hass a toHaveCount()assertion: https://playwright.dev/docs/api/class-locatorassertions#locator-assertions-to-have-count

Suggested solution

Introduce a similar toHaveCount assertion in vitest, so that we can instead write

await expect.element(locator).toHaveCount(3);

Alternative

No response

Additional context

No response

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