This provides a consistent global starting state for tests that interact with admin screens.
Individual tests no longer need to invoke `set_current_screen( 'front' )` (or an alternative implementation) as a reset.
Follow-up to [29251], [29860], [31046], [36721], [38678], [48908], [50433].
Props hellofromTonya, johnbillion.
Fixes#53431.
git-svn-id: https://develop.svn.wordpress.org/trunk@51419 602fd350-edb4-49c9-b593-d223f7449a82