Update zlib test to use separate file for flock()

This should prevent the nightly failures that fail with permission
denied on platforms with mandatory locks.

Closes phpGH-20351.

Author: ndossche

ext/zlib/tests/zlib_flock.txt.gz

@@ -4,7 +4,7 @@ Test function stream_get_meta_data on a zlib stream
 zlib
 --FILE--
 <?php
-$f = __DIR__."/004.txt.gz";
+$f = __DIR__."/zlib_flock.txt.gz";
 $h = gzopen($f,'r');
 var_dump(flock($h, LOCK_SH));
 gzclose($h);