feature #43671 add ResponseIsUnprocessable (garak)

This PR was squashed before being merged into the 5.4 branch.

Discussion
----------

add ResponseIsUnprocessable

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #40894
| License       | MIT
| Doc PR        | none

Commits
-------

61df7f1752 add ResponseIsUnprocessable

Author: fabpot

Test/BrowserKitAssertionsTrait.php

@@ -94,6 +94,11 @@ public static function assertResponseCookieValueSame(string $name, string $expec
         ), $message);
     }
 
+    public static function assertResponseIsUnprocessable(string $message = ''): void
+    {
+        self::assertThatForResponse(new ResponseConstraint\ResponseIsUnprocessable(), $message);
+    }
+
     public static function assertBrowserHasCookie(string $name, string $path = '/', string $domain = null, string $message = ''): void
     {
         self::assertThatForClient(new BrowserKitConstraint\BrowserHasCookie($name, $path, $domain), $message);