minor symfony#57994 [Mime] fix test (xabbuh)

This PR was merged into the 6.4 branch.

Discussion
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[Mime]  fix test

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

updates the test added in symfony#57892 to reflect the change of behavior introduced with symfony#52033

Commits
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2e8b9ce fix test

Author: xabbuh

src/Symfony/Component/Mime/Tests/Part/Multipart/FormDataPartTest.php

@@ -244,11 +244,13 @@ public function testBoundaryContentTypeHeader()
         );
     }
 
-    public function testConstructThrowsOnUnexpectedFieldType()
+    public function testGetPartsThrowsOnUnexpectedFieldType()
     {
+        $dataPart = new FormDataPart(['foo' => new \stdClass()]);
+
         $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage('A form field value can only be a string, an array, or an instance of TextPart ("stdClass" given).');
+        $this->expectExceptionMessage('The value of the form field "foo" can only be a string, an array, or an instance of TextPart, "stdClass" given.');
 
-        new FormDataPart(['foo' => new \stdClass()]);
+        $dataPart->getParts();
     }
 }