minor #38557 Fix tests (jderusse)

This PR was merged into the 4.4 branch.

Discussion
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Fix tests

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | /
| License       | MIT
| Doc PR        | /

Introduced by #38551

Commits
-------

91122492e8 Fix tests

src/Symfony/Component/HttpClient/Tests/Response/MockResponseTest.php

@@ -41,12 +41,6 @@ class MockResponseTest extends TestCase
             'message' => 'Response body is empty.',
         ];
 
-        yield [
-            'content' => '{}',
-            'responseHeaders' => ['content-type' => 'plain/text'],
-            'message' => 'Response content-type is "plain/text" while a JSON-compatible one was expected for "https://example.com/file.json".',
-        ];
-
         yield [
             'content' => 'not json',
             'responseHeaders' => [],