Tests: Move the tests for `get_the_modified_time()` to a more appropriate place.

Add some new tests to better cover the functionality, for consistency with `get_the_date()` and `get_the_time()`.

Follow-up to [48911], [48912], [48918].

Props wittich.
Merges [48924] to the 5.5 branch.
Fixes #51184.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48688 1a063a9b-81f0-0310-95a4-ce76da25c4cd