zlib: fix test skipif sections

Depending on the specific PHP configuration, the regex matching for the
version number in get_zlib_version() can fail.
On master, this results in deprecation warnings and subsequent test
borks:
```
Deprecated: version_compare(): Passing null to parameter #1 ($version1) of type string is deprecated in /work/php-src/ext/zlib/tests/gzgetc_basic.skip.php on line 3 [/work/php-src/ext/zlib/tests/gzgetc_basic.phpt]
Deprecated: version_compare(): Passing null to parameter #1 ($version1) of type string is deprecated in /work/php-src/ext/zlib/tests/gzgetc_basic_1.skip.php on line 4
```

Since ZLIB_VERSION is a constant containing the version as a string,
remove all this nonsense and just use the constant directly.

Author: ndossche

ext/zlib/tests/bug55544-win.phpt

@@ -9,11 +9,8 @@ if( substr(PHP_OS, 0, 3) != "WIN" ) {
   die("skip.. only for Windows");
 }
 
-
-
-include 'func.inc';
-if (version_compare(get_zlib_version(), "1.2.11") < 0) {
-    die("skip - at least zlib 1.2.11 required, got " . get_zlib_version());
+if (version_compare(ZLIB_VERSION, "1.2.11") < 0) {
+    die("skip - at least zlib 1.2.11 required, got " . ZLIB_VERSION);
 }
 ?>
 --FILE--

ext/zlib/tests/func.inc

@@ -4,9 +4,7 @@ Test function gzgetc() by calling it with its expected arguments zlib 1.2.5
 zlib
 --SKIPIF--
 <?php
-
-include 'func.inc';
-if (version_compare(get_zlib_version(), '1.2.5') > 0) {
+if (version_compare(ZLIB_VERSION, '1.2.5') > 0) {
     die('skip - only for zlib <= 1.2.5');
 }
 ?>

ext/zlib/tests/gzencode_variation2-win32.phpt

@@ -4,9 +4,7 @@ Test function gzgetc() by calling it with its expected arguments zlib 1.2.7
 zlib
 --SKIPIF--
 <?php
-
-include 'func.inc';
-if (version_compare(get_zlib_version(), '1.2.7') < 0) {
+if (version_compare(ZLIB_VERSION, '1.2.7') < 0) {
     die('skip - only for zlib >= 1.2.7');
 }
 ?>