Tests: Use `assertSame()` in `get_adjacent_post()` tests. #25683

Job	Run time
QUnit Tests / Run QUnit tests	2m 21s
Slack Notifications / Prepare notifications	4s
Slack Notifications / Success notifications	5s
Slack Notifications / Failure notifications	0s
Slack Notifications / Cancelled notifications	0s
Slack Notifications / Fixed notifications	0s
Failed workflow tasks	-1s
	2m 29s

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