Fix test_foreign.py OPTIONS tests so they don't depend on the order of options.

 * tests/dbobject/test_foreign.py
   (ForeignDataWrapperToSqlTestCase.test_alter_wrapper_options,
   ForeignServerToSqlTestCase.test_add_server_options,
   UserMappingToSqlTestCase.test_drop_user_mapping_options): Split
   options and order them before verification.