Summary:
This diff adds another complexity level for Stacking Context and zIndex tests: Now we test not only "noting-to-something" cases but also cases where we apply changes to an already non-empty tree, validating our assumption of each step.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D25623176
fbshipit-source-id: 29a447d93522a2a4e7912351f907c46ac7b218bb