Fix leaky test:

- Fix rails#51347
- The tests in this suite are leaky which results in other tests
  from other suite from sometimes failing.

  The problem is due to modifying some database tables while
  a prepared statement on Postgreql already exists. This causes
  Postgres to report an error
  "cached plan must not change result type".

  The order of reproduction is as follow:

  - We have more than one connection in the pool (Let's say Conn1 and
   Conn2 for the sake of the example)
  - A first test runs a query, Conn1 is used, we prepare a statement.
  - The leaky test modify some table on Conn2.
  - A third test runs a query, Conn1 is used, the query was prepared.
    Postgresql reports the  "cached plan must not change result type".

  This commis fixes that by clearing the statement cache on each
  connection.

Author: Edouard-chin

activerecord/test/cases/migration_test.rb

@@ -61,12 +61,14 @@ def setup
       Thing.lease_connection.drop_table(table) rescue nil
     end
     Thing.reset_column_information
+    clear_statement_cache(Thing)
 
     %w(reminders people_reminders prefix_reminders_suffix).each do |table|
       Reminder.lease_connection.drop_table(table) rescue nil
     end
     Reminder.reset_table_name
     Reminder.reset_column_information
+    clear_statement_cache(Reminder)
 
     %w(last_name key bio age height wealth birthday favorite_day
        moment_of_truth male administrator funny).each do |column|
@@ -76,6 +78,7 @@ def setup
     Person.lease_connection.remove_column("people", "middle_name") rescue nil
     Person.lease_connection.add_column("people", "first_name", :string)
     Person.reset_column_information
+    clear_statement_cache(Person)
 
     ActiveRecord::Migration.verbose = @verbose_was
   end
@@ -1131,6 +1134,12 @@ def self.base_class; self; end
       }
     end
 
+    def clear_statement_cache(model)
+      model.connection_handler.each_connection_pool do |pool|
+        pool.connections.each(&:clear_cache!)
+      end
+    end
+
     def with_another_process_holding_lock(lock_id)
       thread_lock = Concurrent::CountDownLatch.new
       test_terminated = Concurrent::CountDownLatch.new