Adds a `public` visibility to test fixtures, tests, data providers, and callbacks methods.
Adds a `private` visibility to helper methods within test classes.
Renames callbacks and helpers that previously started with a `_` prefix. Why? For consistency and to leverage using the method visibility. Further naming standardizations is beyond the scope of this commit.
Props costdev, jrf, hellofromTonya.
Fixes#54177.
git-svn-id: https://develop.svn.wordpress.org/trunk@52010 602fd350-edb4-49c9-b593-d223f7449a82
