E2E Tests: Fix failing image e2e and php unit tests (#36982)

* Fix failing image e2e test by waiting for required element before attempting click
* Switch to waiting for image instead of the image resize buttons in image block e2e tests
* Add delay when clearing input field and typing replacement url in image e2e tests
* Add in wait for user input box to prevent username being partially entered in mentions e2e test
* Add phpunit-polyfills to fix running of php-unit tests locally - related to https://make.wordpress.org/core/2021/09/27/changes-to-the-wordpress-core-php-test-suite/

Co-authored-by: Glen Davies <glen.davies@a8c.com>
Co-authored-by: Andrew Serong <14988353+andrewserong@users.noreply.github.com>
Co-authored-by: Daniel Richards <daniel.richards@automattic.com>

composer.json

@@ -26,7 +26,8 @@
 		"sirbrillig/phpcs-variable-analysis": "^2.8",
 		"wp-phpunit/wp-phpunit": "^5.4",
 		"phpunit/phpunit": "^8.5",
-		"spatie/phpunit-watcher": "^1.23"
+		"spatie/phpunit-watcher": "^1.23",
+		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"require": {
 		"composer/installers": "~1.0"