Tests: Use `assertSame()` in `get_adjacent_post()` tests. #749

Job	Run time
JavaScript type checking / Run JavaScript type checking	1m 37s
Slack Notifications / Prepare notifications	5s
Slack Notifications / Success notifications	5s
Slack Notifications / Cancelled notifications	0s
Slack Notifications / Failure notifications	0s
Slack Notifications / Fixed notifications	0s
Failed workflow tasks	0s
	1m 47s

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