Tests: Move data providers and helpers in `Tests_REST_Server` for consistency.

This ensures that data providers or helper functions used by a single test are located next to the test, for consistency with the rest of the test suite.

Follow-up to [37905], [37943], [45809], [47239], [47260], [47351], [48947], [49252], [49257], [51960], [53110], [56096], [59032].

See #64225.

git-svn-id: https://develop.svn.wordpress.org/trunk@62205 602fd350-edb4-49c9-b593-d223f7449a82