Add e2e test for preview existing post #12895

Job	Run time
Core running from build / Core running from build / Linux	2m 18s
Core running from src / Core running from src / Linux	2m 4s
Core running from build / Core running from build / Windows	4m 14s
Core running from build / Core running from build / MacOS	1m 58s
Core running from src / Core running from src / Windows	4m 19s
Core running from src / Core running from src / MacOS	2m 52s
Gutenberg running from build / Gutenberg running from build / Linux	8m 48s
Gutenberg running from src / Gutenberg running from src / Linux	8m 50s
Gutenberg running from build / Gutenberg running from build / Windows	12m 55s
Gutenberg running from src / Gutenberg running from src / Windows	11m 52s
Gutenberg running from build / Gutenberg running from build / MacOS	9m 21s
Gutenberg running from src / Gutenberg running from src / MacOS	8m 29s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 18m 0s

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