Implement extension to jslib expect library for xk6-browser

[robingustafsson]

We want to start promoting the k6 expect library more for at least functional, contract validation and browser end-to-end testing, but probably also for load testing. Different types of tests need to verify, or expect, different things. In browser end-to-end testing you want to expect things like expect(element).toBeVisible() or expect(page.screenshot()).toMatchSnapshot('referenceScreenshot.js') (using https://github.com/mapbox/pixelmatch).

Here's a non-exhaustive list of expect extensions that we'd want:

• expect(element).toBeChecked()
• expect(element).toBeDisabled()
• expect(element).toBeEditable()
• expect(element).toBeEmpty()
• expect(element).toBeEnabled()
• expect(element).toBeFocused()
• expect(element).toBeHidden()
• expect(element).toBeUnchecked()
• expect(element).toBeVisible()
• expect(element).toContainText(text)
• expect(element).toHaveAttribute(attribute)
• expect(element).toHaveClass(class)
• expect(element).toHaveCount(count)
• expect(element).toHaveCSS(css)
• expect(element).toHaveId(id)
• expect(element).toHaveJSProperty(property)
• expect(element|frame|page).toHaveText(text)
• expect(frame|page).toHaveTitle(title)
• expect(frame|page).toHaveURL(url)
• expect(element).toHaveValue(value)
• expect(element.screenshot()|page.screenshot()|ArrayBuffer).toMatchSnapshot(filename)

Relevant links:

• Playwright code: https://github.com/microsoft/playwright/blob/master/packages/playwright-test/src/matchers/matchers.ts