Build/Test Tools: Check if all the required constants are defined before running the test suite.
Follow-up to [47904].
Props azaozz, TimothyBlynJacobs, SergeyBiryukov.
Fixes #50251.
git-svn-id: https://develop.svn.wordpress.org/trunk@48592 602fd350-edb4-49c9-b593-d223f7449a82