Test deleteOldRevisions.php

Why:

• Maintenance scripts in core are mostly untested and testing them will help to avoid regressions.
  • For example, in testing the script a bug is found where the Maintenance::purgeRedundantText method will throw an unexpected exception if no valid rows exist in the content_address table.

What:

• Create DeleteOldRevisionsTest that fully tests the associated maintenance script.
• Update Maintenance::purgeRedundantText to not attempt to use an empty list as the value provided to IReadableDatabase::getExpr.
  • Add a regression test for this.

Bug: T371167
Change-Id: Id0c3c5d6fe56bc9fe5d5347c82dab9ab61137f58