Build/Test Tools: Add test coverage for the get_posts_navigation() function.
This changeset adds a test case to verify that get_the_posts_navigation() only includes the "Older posts" and "Newer" posts links when appropriate.
Props jongycastillo, michelmany, joyously, geisthanen, SergeyBiryukov, mukesh27, audrasjb, costdev, sun, chrisbaltazar.
Fixes #55751.