Add opaque token test support

[jzheaux]

An application can mock a jwt() authentication in a test using the JwtRequestPostProcessor.

It would be nice to be able to do this for opaque tokens:

this.mvc(get("/").with(opaqueToken()))

this.mvc(get("/").with(opaqueToken()
        .authorities(new SimpleGrantedAuthority("SCOPE_read"))));

this.mvc(get("/").with(opaqueToken()
        .scopes("read")));

[jzheaux]

Related to, though more specific than #7528