minor #13147 [2.3] for consistency, use value of DIRECTORY_SEPARATOR to detect Windows (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[2.3] for consistency, use value of DIRECTORY_SEPARATOR to detect Windows

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

This commit unifies the detection of Windows builds across the Symfony
codebase.

Commits
-------

20a427d use value of DIRECTORY_SEPARATOR to detect Windows

Author: fabpot

Tests/KernelTest.php

@@ -340,7 +340,7 @@ public function doStuff()
 
         // Heredocs are preserved, making the output mixing Unix and Windows line
         // endings, switching to "\n" everywhere on Windows to avoid failure.
-        if (defined('PHP_WINDOWS_VERSION_MAJOR')) {
+        if ('\\' === DIRECTORY_SEPARATOR) {
             $expected = str_replace("\r\n", "\n", $expected);
             $output = str_replace("\r\n", "\n", $output);
         }