minor symfony#58476 [HttpKernel] harden test to not depend on the system's configured default timezone (xabbuh)

This PR was merged into the 5.4 branch.

Discussion
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[HttpKernel] harden test to not depend on the system's configured default timezone

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

fixes https://ci.appveyor.com/project/fabpot/symfony/builds/50740552#L1528

Commits
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e17aad0 harden test to not depend on the system's configured default timezone

Author: xabbuh

src/Symfony/Component/HttpKernel/Tests/HttpCache/ResponseCacheStrategyTest.php

@@ -102,7 +102,7 @@ public function testMaxAgeUpdatedFromExpiresHeader()
         $cacheStrategy = new ResponseCacheStrategy();
 
         $response1 = new Response();
-        $response1->setExpires(new \DateTime('+ 1 hour'));
+        $response1->setExpires(new \DateTime('+ 1 hour', new \DateTimeZone('UTC')));
         $response1->setPublic();
         $cacheStrategy->add($response1);
 
@@ -112,15 +112,15 @@ public function testMaxAgeUpdatedFromExpiresHeader()
 
         $this->assertSame('3600', $response->headers->getCacheControlDirective('max-age'));
         $this->assertNull($response->headers->getCacheControlDirective('s-maxage'));
-        $this->assertSame((new \DateTime('+ 1 hour'))->format('D, d M Y H:i:s').' GMT', $response->headers->get('Expires'));
+        $this->assertSame((new \DateTime('+ 1 hour', new \DateTimeZone('UTC')))->format('D, d M Y H:i:s').' GMT', $response->headers->get('Expires'));
     }
 
     public function testMaxAgeAndSharedMaxAgeUpdatedFromExpiresHeader()
     {
         $cacheStrategy = new ResponseCacheStrategy();
 
         $response1 = new Response();
-        $response1->setExpires(new \DateTime('+ 1 day'));
+        $response1->setExpires(new \DateTime('+ 1 day', new \DateTimeZone('UTC')));
         $response1->setPublic();
         $cacheStrategy->add($response1);
 
@@ -131,7 +131,7 @@ public function testMaxAgeAndSharedMaxAgeUpdatedFromExpiresHeader()
 
         $this->assertSame('3600', $response->headers->getCacheControlDirective('max-age'));
         $this->assertSame('86400', $response->headers->getCacheControlDirective('s-maxage'));
-        $this->assertSame((new \DateTime('+ 1 hour'))->format('D, d M Y H:i:s').' GMT', $response->headers->get('Expires'));
+        $this->assertSame((new \DateTime('+ 1 hour', new \DateTimeZone('UTC')))->format('D, d M Y H:i:s').' GMT', $response->headers->get('Expires'));
     }
 
     public function testMainResponseNotCacheableWhenEmbeddedResponseRequiresValidation()