E2E Test Utils: Create standard set of lifecycle utilities

[aduth]

Related: #22712

Background:

In end-to-end tests, it's quite common to need to establish some temporary environment state relevant for the duration of that test file. For example, a plugin should be activated while a test is being run. In code, this usually presents itself as a pair of beforeEach and afterEach (or beforeAll and afterAll) Jest setup and teardown methods, containing the "enable"/"activate" and "disable"/"deactivate" steps respectively.

Evidenced by #22712, this can be error-prone for a couple reasons:

• It assumes that the developer understands that side effects from activation steps can linger beyond the test file and bleed into other tests, such that they take the action to implement the tear-down.
• In cases where arguments must be provided, it requires that the arguments be provided consistently (prone to typos, etc). This can be especially problematic when attempting to "unset" a temporary state to the default value, where if assigned as an assumed literal value, it's likely to break if that default ever changes, since there's no single source of truth (example, where "large" is the assumed default)

Another disadvantage is that in lieu of setting the expectation that a developer implement this themselves, many common "state" are reset universally after every test, regardless of whether it's expected the state was modified for the test (example).

Proposal:

The changes in #22712 present a possible pattern for utilities which implement the full lifecycle of a testing state, taking advantage of the fact that Jest lifecycle methods can be called from utilities, not exclusively within the test itself.

Thus, instead of the general pattern...

beforeAll( () => {
  activateFoo( 'bar' );
} );

afterAll( () => {
  deactivateFoo( 'bar' );
} );

...it can be expressed as:

useFoo( 'bar' );

...where useFoo is implemented as:

function useFoo( arg ) {
  beforeAll( () => {
    activateFoo( arg );
  } );

  afterAll( () => {
    deactivateFoo( arg );
  } );
}

Task:

Implement full lifecycle utilities for common end-to-end test state:

• activatePlugin, deactivatePlugin → usePlugin
  • Example
• setBrowserViewport → useBrowserViewport
  • Example
• setPostFormat → usePostFormat
  • Example
• setUpResponseMocking → useResponseMocks
  • Example

[aduth]

Some relevant discussion during the Weekly JavaScript Chat on June 2 (link requires registration):

https://wordpress.slack.com/archives/C5UNMSU4R/p1591108681382000