Move TestingAuthenticationToken and TestingAuthenticationProvider to test package

[HerrDerb]

Summary

The classes TestingAuthenticationToken and TestingAuthenticationProvider are currently located inside of the productive package org.springframework.security.authentication.

As stated in both of the classes, these classes are for testing purposes and therefore should be moved to the corresponding test package.

TestingAuthenticationToken
 * An {@link org.springframework.security.core.Authentication} implementation that is
 * designed for use whilst unit testing.

TestingAuthenticationProvider
 * The purpose of this implementation is to facilitate unit testing. This provider should
 * <b>never be enabled on a production system</b>.