Fix failing tests introduced by rails#41030

In rails#41030 the tests didn't show as failing because they only fail when
run in isolation which happens on the merge to master.

In this test we need to re-establish the connection to
`ActiveRecord::Base` or else the tests get the connections confused and
for some reason some of the migration tests were looking for the wrong
connection specification name.

Author: eileencodes

activerecord/test/cases/connection_adapters/connection_swapping_nested_test.rb

@@ -434,6 +434,7 @@ def test_application_record_prevent_writes_can_be_changed
           end
         ensure
           Object.send(:remove_const, :ApplicationRecord)
+          ActiveRecord::Base.establish_connection :arunit
         end
       end
     end