[Mailer] Add EmailSubjectContains constraint (#49939)

Adds assertEmailSubjectContains and assertEmailSubjectNotContains

Author: JohJohan

Test/MailerAssertionsTrait.php

@@ -90,6 +90,16 @@ public static function assertEmailAddressContains(RawMessage $email, string $hea
         self::assertThat($email, new MimeConstraint\EmailAddressContains($headerName, $expectedValue), $message);
     }
 
+    public static function assertEmailSubjectContains(RawMessage $email, string $expectedValue, string $message = ''): void
+    {
+        self::assertThat($email, new MimeConstraint\EmailSubjectContains($expectedValue), $message);
+    }
+
+    public static function assertEmailSubjectNotContains(RawMessage $email, string $expectedValue, string $message = ''): void
+    {
+        self::assertThat($email, new LogicalNot(new MimeConstraint\EmailSubjectContains($expectedValue)), $message);
+    }
+
     /**
      * @return MessageEvent[]
      */

Tests/Functional/MailerTest.php

@@ -104,6 +104,8 @@ public function testMailerAssertions()
         $this->assertEmailAttachmentCount($email, 1);
 
         $email = $this->getMailerMessage($second);
+        $this->assertEmailSubjectContains($email, 'Foo');
+        $this->assertEmailSubjectNotContains($email, 'Bar');
         $this->assertEmailAddressContains($email, 'To', '[email protected]');
         $this->assertEmailAddressContains($email, 'To', '[email protected]');
         $this->assertEmailAddressContains($email, 'Reply-To', '[email protected]');