Tests: Correct hardcoded "3 months ago" assertion in...

Tests: Correct hardcoded "3 months ago" assertion in `tests/rest-api/rest-block-directory-controller.php`.

Now that more than 3 months have passed since 2020-03-23, this was causing test failures.

Follow-up to [48242].

See #50321.
Built from https://develop.svn.wordpress.org/trunk@48333


git-svn-id: https://core.svn.wordpress.org/trunk@48102 1a063a9b-81f0-0310-95a4-ce76da25c4cd