Build/Test tools: Introduce partial unit tests for WP_Upgrader.
WP_Upgrader
Props jipmoors, karlijnbk, chaion07, cu121, martin.krcho, costdev, mukesh27, hellofromTonya, SergeyBiryukov, audrasjb, jrf. Merges [56992] to the 6.4 branch. Fixes #54245.