Add oidcLogin MockMvc Test Support

[jzheaux]

Related to #6634.

It'd be handy to have test support for oidcLogin:

this.mvc.perform(get("/").with(oidcLogin()));

or

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

The result would be an OAuth2AuthenticationToken as well as an OAuth2AuthorizedClient.

Ideally, tests would work with controllers using @AuthenticatedPrincipal as well as @RegisteredOAuth2AuthorizedClient annotations.