minor #6736 Use message argument for PHPunit assert() functions (SimonHeimberg, WouterJ)

This PR was merged into the 2.7 branch.

Discussion
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Use message argument for PHPunit assert() functions

Finishes #6118

Original PR description:

 > Assert functions from PHPUnit do have an optional message argument. This is especially usefull for assertTrue because the message printed on failure does not (and can not) contain any details. Demonstrate this possibility

Commits
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d604bfd Make messages positive
97e072e use the message argument of assertX functions

Author: wouterj

book/testing.rst

@@ -263,7 +263,8 @@ document::
             $client->getResponse()->headers->contains(
                 'Content-Type',
                 'application/json'
-            )
+            ),
+            'the "Content-Type" header is "application/json"' // optional message shown on failure
         );
 
         // Assert that the response content contains a string
@@ -272,7 +273,7 @@ document::
         $this->assertRegExp('/foo(bar)?/', $client->getResponse()->getContent());
 
         // Assert that the response status code is 2xx
-        $this->assertTrue($client->getResponse()->isSuccessful());
+        $this->assertTrue($client->getResponse()->isSuccessful(), 'response status is 2xx');
         // Assert that the response status code is 404
         $this->assertTrue($client->getResponse()->isNotFound());
         // Assert a specific 200 status code
@@ -283,7 +284,8 @@ document::
 
         // Assert that the response is a redirect to /demo/contact
         $this->assertTrue(
-            $client->getResponse()->isRedirect('/demo/contact')
+            $client->getResponse()->isRedirect('/demo/contact'),
+            'response is a redirect to /demo/contact'
         );
         // ...or simply check that the response is a redirect to any URL
         $this->assertTrue($client->getResponse()->isRedirect());