minor #19765 [Config] Fix DirectoryResourceTest for symlinks (ogizanagi)

This PR was merged into the 3.1 branch.

Discussion
----------

[Config] Fix DirectoryResourceTest for symlinks

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | no (only tests, locally)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Tests were failing locally (on my macbook), because the `DirectoryResource` implementation now uses `realpath`.
Which means the path returned by `->getResource()` may differ from the given one in case it is a symlink:

```diff
There were 2 failures:

1) Symfony\Component\Config\Tests\Resource\DirectoryResourceTest::testGetResource
->getResource() returns the path to the resource
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-/var/folders/j0/grc5nstx5b5f95cgg09_cy_h0000gn/T/symfonyDirectoryIterator
+/private/var/folders/j0/grc5nstx5b5f95cgg09_cy_h0000gn/T/symfonyDirectoryIterator

src/Symfony/Component/Config/Tests/Resource/DirectoryResourceTest.php:56

2) Symfony\Component\Config\Tests\Resource\DirectoryResourceTest::testSerializeUnserialize
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-/var/folders/j0/grc5nstx5b5f95cgg09_cy_h0000gn/T/symfonyDirectoryIterator
+/private/var/folders/j0/grc5nstx5b5f95cgg09_cy_h0000gn/T/symfonyDirectoryIterator

src/Symfony/Component/Config/Tests/Resource/DirectoryResourceTest.php:169
```

Commits
-------

56f74eb [Config] Fix DirectoryResourceTest for symlinks

Author: fabpot

src/Symfony/Component/Config/Tests/Resource/DirectoryResourceTest.php

@@ -53,7 +53,7 @@ protected function removeDirectory($directory)
     public function testGetResource()
     {
         $resource = new DirectoryResource($this->directory);
-        $this->assertSame($this->directory, $resource->getResource(), '->getResource() returns the path to the resource');
+        $this->assertSame(realpath($this->directory), $resource->getResource(), '->getResource() returns the path to the resource');
     }
 
     public function testGetPattern()
@@ -166,7 +166,7 @@ public function testSerializeUnserialize()
 
         $unserialized = unserialize(serialize($resource));
 
-        $this->assertSame($this->directory, $resource->getResource());
+        $this->assertSame(realpath($this->directory), $resource->getResource());
         $this->assertSame('/\.(foo|xml)$/', $resource->getPattern());
     }