[retry] implement waitFor method

[spalger]

We currently use the retry service to call a function over and over in a loop, waiting for it to run without throwing an error, and ultimately failing if it does not succeed before a timeout is exceeded. This is the easiest way to get certain interactions to work, either because we don't know when we should be able to execute the interaction successfully, or because the timing is just too tricky to plan out correctly. Another place where we are using the retry service, which I don't think is appropriate, is when we need to wait for a certain condition to be met. This is where retry.waitFor() comes in:

await retry.waitFor('dashboard search to be enabled', async () => {
  const searchInput = await testSubjects.find('savedObjectFinderSearchInput');
  return await searchInput.isEnabled();
})

The retry.waitFor() method behaves much like the retry.try() method behind the scenes, calling a function over and over, but instead of waiting for it to run without throwing an error it waits for it to return a "truthy" value. It also requires a description string that is used to make rather nice log output and a more descriptive error message than something like https://github.com/elastic/kibana/blob/master/test/functional/apps/dashboard/_data_shared_attributes.js#L61-L67

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[LeeDr]

Would retry.waitFor work with expect statements? We don't return await those. But they're a very common thing we would want to wait for.

[spalger]

waitFor replaces these types of expect statements:

retry.try(async () => {
  // ...
  expect(someBoolean).to.be(true)
})

with

retry.waitFor('some boolean', async () => {
  // ...
  return someBoolean
})

Retry's that are checking other things might be converted like so:

retry.waitFor('all visualizations to render', async () => {
  const renderedCount = await PageObjects.dashboard.countRenderedVisualizations();
  return renderedCount === expectedAmount
})

using waitFor here instead of retry.try() will mostly be beneficial for the better logging and readability of the code.

[spalger]

You can see an example implementation that's actually improving the stability of the tests in #21629 here: https://github.com/elastic/kibana/blob/7cb23e9d1dd4aeba3c6869b3b6ddbd93e58c74c6/test/functional/page_objects/dashboard_page.js#L179-L181

[LeeDr]

LGTM - code reviewed only. Passing in Jenkins.

[spalger]

6.5/6.x: 0072315
6.4: a6296dc