Add async check util

[allansson]

What

This PR adds a version of the check function that makes working with async/await more ergonomic. It is drop-in compatible with the existing check function, i.e. using only sync values won't wrap results in a Promise. As an added bonus, it's also possible to skip passing the first value.

Why

Writing clean code using the standard check function is hard when using async code. To check a value you need to declare a temporary variable and await the value first:

const myVar = await res.json()

check(myVar, {
  'is Admin': () => myVar.isAdmin
})

This clutters the code with single use declarations and creates the need to make up bogus variable names.

Examples

// Sync
const a = check(res, {
  'as value': res.status === 200,
  'as func': res => res.headers['Content-Type'] === "application/json"
})

// Async
const b = await check(locator, {
  'as promise': locator.isChecked(),
  'as async func': l => l.getAttribute("draggable")
})

// Mixed
const c = await check(res, {
  'as value': true,
  'as promise': Promise.resolve(false)
})

EDIT: Removed the shorthand version because it wouldn't work with tags and having it as a drop-in replacement is more important.

Issue: grafana/jslib.k6.io#125

[inancgumus]

Nice 🎉

[going-confetti]

LGTM!