Just do it the usual way and then insert it using SecurityContextHolder.setContext()
in your test class, for example:
Controller:
Authentication a = SecurityContextHolder.getContext().getAuthentication();
Test:
Authentication authentication = Mockito.mock(Authentication.class);
// Mockito.whens() for your authorization object
SecurityContext securityContext = Mockito.mock(SecurityContext.class);
Mockito.when(securityContext.getAuthentication()).thenReturn(authentication);
SecurityContextHolder.setContext(securityContext);