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

This ensures that not only the return values match the expected results, but also that their type is the same.

Going forward, stricter type checking by using `assertSame()` should generally be preferred to `assertEquals()` where appropriate, to make the tests more reliable.

Follow-up to [60733], [61066].

Props sagardeshmukh.
See #64324.

git-svn-id: https://develop.svn.wordpress.org/trunk@62247 602fd350-edb4-49c9-b593-d223f7449a82