invoke_callback_2 fails because of ArgumentCountException (init_cb_arg gets only 2 args)
Check number of passed arguments with func_num_args() insteadof checking $args is null
session_lock fails because of second warning in 7.1
Skip session_lock.phpt for php < 7.1 and create new testcase for >= 7.1 which tests both warnings.
Update .travis.yml, don't allow failures for 7.1 anymore!