Build/Test Tools: Correctly set up the current screen during list table tests...

Build/Test Tools: Correctly set up the current screen during list table tests so that they don't fail when run individually.

Props desrosj
Fixes #38761

Built from https://develop.svn.wordpress.org/trunk@39481


git-svn-id: https://core.svn.wordpress.org/trunk@39421 1a063a9b-81f0-0310-95a4-ce76da25c4cd