Remove the item type from the collection declaration, you'll get a warning but the test will work.
@Test
public void testFoo()
{
// setup
Authentication mockAuthentication = createMock(Authentication.class);
Collection authorities = Collections.emptyList();
expect(mockAuthentication.getAuthorities()).andReturn(authorities);
// exercise
EasyMock.replay(mockAuthentication);
Collection<? extends GrantedAuthority> retAuth = mockAuthentication.getAuthorities();
// verify
EasyMock.verify(mockAuthentication);
assertEquals(authorities, retAuth);
}