Migrate Cover Block tests to Playwright (#45784)

* Add new Playwright E2E tests for the cover block.

* Add comment

* Update block list selector

* test/e2e/specs/editor/blocks/cover.spec.js
- update spec with accessibility-based selectors where possible.

* test/e2e/specs/editor/blocks/cover.spec.js
- rebase, rebuild and update the selectors

* packages/e2e-tests/specs/editor/blocks/cover.test.js
- remove Puppeteer file.

* test/e2e/specs/editor/blocks/cover.spec.js
- clarify use of `span` element in the first spec.

* - add comment explaing use of CSS selector for '.components-resizable-box__handle-bottom'.
- move image upload and filename methods previously handled by `getTestImage` into a POM-like function at bottom of file.

* - remove testing `test.only`

* - define the coverBlock locator after inserting the block, and use that locator as starting point for all other interactions.
- use `getComputedStyle` instad of `node.style.<attribute>`.
- replace implementation detail check on image upload with a `expect.toPass` check.
- remove steps when resizing box.
- replace implementation detail check for coverbox height with truthy check.

* - fix the import to use `import`.
- add comment explaining the use of `span[aria-hidden="true"]`.

* - anchor selectors on a parent selector.

* - use the Cover block's resize handle as basis for calculation when resizing the block.

* - remove testing "only"

* - check the src repeatedly to contain the fileBasename instead.

* - move `getBackgroundColorAndOpacity` into the ImageBlockUtils POM.

* Rename POM to CoverBlockUtils

Author: worldomonation