kunit: string-stream-test: use KUNIT_DEFINE_ACTION_WRAPPER

Use KUNIT_DEFINE_ACTION_WRAPPER macro to define the 'kfree' and
'string_stream_destroy' wrappers for kunit_add_action.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Acked-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>