Re-apply: MCR: Set testwiki to use the new MCR-only schema
This is a re-submit of I43df1aa93c473bbbcd916f1b866f134a2e410f45.
It reverts commit 5593e910aa948954265288c785d9a07185037823.
Reason: After T236735 has been fixed, let's try again to set
testwiki to SCHEMA_COMPAT_NEW for the revision schema.
Bug: T198558
Change-Id: I4d9f6e226382be5f6cfa77c4aa05ec215f448aea