ci: Fix OOM failures in header_over_4gib.phpt

The test keeps a header larger than 4 GiB alive while sapi_header_op()
duplicates it, so its peak memory usage exceeds 8 GiB. The 6 GiB FreeBSD
CI VM can kill either the test process (Termsig=9) or a test worker.

Skip the test on FreeBSD, following ext/bz2/tests/gh20807.phpt. Run it in
isolation with --CONFLICTS-- all so other tests do not add to its memory
pressure on the remaining platforms.

Author: LamentXU123

ext/standard/tests/general_functions/header_over_4gib.phpt

@@ -5,7 +5,10 @@ header() with a header line longer than 4 GiB
 if (!getenv('RUN_RESOURCE_HEAVY_TESTS')) die('skip resource-heavy test');
 if (PHP_INT_SIZE != 8) die('skip this test is for 64bit platforms only');
 if (getenv('SKIP_SLOW_TESTS')) die('skip slow tests excluded by request');
+if (PHP_OS === 'FreeBSD') die('skip Worker does not handle OOM gracefully');
 ?>
+--CONFLICTS--
+all
 --INI--
 memory_limit=-1
 --FILE--