Change Detection: Add a test case for post trashing.

[epiqueras]

Follows #18275

Description

This PR fixes and reintroduces the test case from #18275.

It also replaces some ad-hoc saving code with the correct e2e test util, saveDraft.

How has this been tested?

The new tests were verified to test for the issue fixed in #18275.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[aduth]

There's a (likely related) end-to-end test failure:

FAIL packages/e2e-tests/specs/editor/various/change-detection.test.js (60.563s)
  ● Change detection › should not prompt to confirm unsaved changes when trashing an existing post
    expect(jest.fn()).not.toHaveErrored(expected)
    Expected mock function not to be called but it was called with:
    ["Failed to load resource: the server responded with a status of 500 (Internal Server Error)"]
      at Object.expect (../jest-console/build/@wordpress/jest-console/src/index.js:34:4)
          at runMicrotasks (<anonymous>)

[aduth]

From your comment at #18275 (comment) :

This works because refreshing a page with a dialog doesn't get rid of the dialog. So if navigation didn't happen because the dialog stopped it, assertIsDirty wouldn't assert false.

But the 'dialog' event handler isn't added until assertIsDirty is called, so if the dialog was already present by the time we reach this line, the event callback would never be triggered.

[epiqueras]

True! I missed that.

[epiqueras]

Fixed!

[aduth]

👍