Report test as risky, if skipif does not has standard-output side-effect

[staabm]

since #5998 we know which side-effects the PHPT skipif code has.

A skipif condition is expected to output a string starting with "skip", in case the test should be skipped.
this also means a SKIPIF condition which does not at all produce output, can never have a "skipping-effect" on the test-code.

-> we could report a test as "risky" when SKIPIF is declared but does not have such a expected standard output side-effect. I remember we talked about marking such test as "error" instead of "risky", but decided against it because some people use the skipif section to prepare something for the test-case (without the intention to skip it)

--TEST--
PHPT with --SKIPIF-- but without standard output side-effect is risky
--INI--
error_reporting=-1
--SKIPIF--
<?php declare(strict_types=1);
if (1 == 2) {
   exit(1);
}
--FILE--
<?php declare(strict_types=1);
echo "Hello, World!\n";
--EXPECT--
Hello, World!