minor #1529 Added test of FormTrait::namedForm() (chihiro-adachi)

This PR was squashed before being merged into the 2.1.x-dev branch (closes #1529).

Discussion
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Added test of FormTrait::namedForm()

I did not see the test of FormTrait::namedForm(), so I created it.

Commits
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32640626 Added test of FormTrait::namedForm()

tests/Silex/Tests/Application/FormTraitTest.php

@@ -13,6 +13,7 @@ namespace Silex\Tests\Application;
 
 use PHPUnit\Framework\TestCase;
 use Silex\Provider\FormServiceProvider;
+use Symfony\Component\Form\FormBuilder;
 
 /**
  * FormTrait test cases.
@@ -23,7 +24,15 @@ class FormTraitTest extends TestCase
 {
     public function testForm()
     {
-        $this->assertInstanceOf('Symfony\Component\Form\FormBuilder', $this->createApplication()->form());
+        $this->assertInstanceOf(FormBuilder::class, $this->createApplication()->form());
+    }
+
+    public function testNamedForm()
+    {
+        $builder = $this->createApplication()->namedForm('foo');
+
+        $this->assertInstanceOf(FormBuilder::class, $builder);
+        $this->assertSame('foo', $builder->getName());
     }
 
     public function createApplication()