Run headers_without_mbstring.phpt instead of always skipping it

The test skips whenever ext/mbstring is present, which is every job in the CI
matrix, so the non-mbstring header encoding path had no coverage at all. That
is how its expectations came to encode a chunking bug unnoticed.

Add an --INI-- section disabling mb_substr()/mb_strlen(). Unlike the -i
command line flag, --INI-- also applies to --SKIPIF--, so the existing skip
condition now acts as a fallback: the test runs when the functions can be
disabled, and still skips if they cannot.

Author: sebastka

tests/headers_without_mbstring.phpt

@@ -1,5 +1,7 @@
 --TEST--
 Multi-test for headers encoding using base64 and quoted-printable
+--INI--
+disable_functions=mb_substr,mb_strlen
 --SKIPIF--
 <?php
 if (function_exists('mb_substr') && function_exists('mb_strlen')) {