Disabled prepared statements in test

Some tests may test migrations and change the types of columns. If this
happens, Rails may cache a statement that will cause PostgreSQL to fail with
the message, "cached plan must not change result type":
https://github.com/rails/rails/issues/12330

This happened in https://gitlab.com/gitlab-org/gitlab-ee/issues/2890#note_34636077.

config/database.yml.mysql

@@ -42,3 +42,4 @@ test: &test
   password:
   # host: localhost
   # socket: /tmp/mysql.sock
+  prepared_statements: false

config/database.yml.postgresql

@@ -46,3 +46,4 @@ test: &test
   username: postgres
   password:
   # host: localhost
+  prepared_statements: false