Test json format on failed platform reqs (#11477)

* test: Adds case for json format arg

* test: A failed platofrm requirement

* chore: style fix

* chore: removes redundant test case

tests/Composer/Test/Command/CheckPlatformReqsCommandTest.php

@@ -88,4 +88,55 @@ public static function caseProvider(): \Generator
             "Checking platform requirements using the lock file\next-barbaz 2.3.4.5   success \next-foobar 2.3.4     success"
         ];
     }
+
+    public function testFailedPlatformRequirement(): void 
+    {
+        $this->initTempComposer([
+            'require' => [
+                'ext-foobar' => '^0.3'
+            ],
+            'require-dev' => [
+                'ext-barbaz' => '^2.3'
+            ]
+        ]);
+
+        $packages = [
+            self::getPackage('ext-foobar', '2.3.4'),
+        ];
+        $devPackages = [
+            self::getPackage('ext-barbaz', '2.3.4.5')
+        ];
+
+        $this->createInstalledJson($packages, $devPackages);
+
+        $this->createComposerLock($packages, $devPackages);
+
+        $appTester = $this->getApplicationTester();
+        $appTester->run(['command' => 'check-platform-reqs', '--format' => 'json']);
+
+        $expected = 'Checking platform requirements for packages in the vendor dir
+[
+    {
+        "name": "ext-barbaz",
+        "version": "2.3.4.5",
+        "status": "success",
+        "failed_requirement": null,
+        "provider": null
+    },
+    {
+        "name": "ext-foobar",
+        "version": "2.3.4",
+        "status": "failed",
+        "failed_requirement": {
+            "source": "__root__",
+            "type": "requires",
+            "target": "ext-foobar",
+            "constraint": "^0.3"
+        },
+        "provider": null
+    }
+]';
+
+        $this->assertSame(trim($expected), trim($appTester->getDisplay(true)));
+    }
 }