Fix with_revert spec helper

This works with either `adapter.materialize_transactions` before
the savepoint creation or by skipping the savepoint entirely and
reverting the migration by raising `ActiveRecord::Rollback`

Close #179

Ref: rails/rails@57bc28f rails/rails#44576

spec/support/time_machine/helpers.rb

@@ -13,6 +13,7 @@ def adapter
 
     def with_revert
       adapter.transaction do
+        adapter.materialize_transactions if adapter.respond_to?(:materialize_transactions)
         adapter.create_savepoint 'revert'
 
         yield