Issue #843: Correct a failed Travis build by excluding a PHPCS rule.

There was an error:
Class file names should be based on the class name with 'class-'
But the format of the other test files is different.
So use that format, and exclude this rule for test files.

phpcs.xml

@@ -11,6 +11,9 @@
 		</properties>
 	</rule>
 
+	<rule ref="WordPress.Files.FileName.InvalidClassFileName">
+		<exclude-pattern>tests/*</exclude-pattern>
+	</rule>
 	<rule ref="WordPress.Arrays.MultipleStatementAlignment.LongIndexSpaceBeforeDoubleArrow">
 		<exclude-pattern>tests/test-tag-and-attribute-sanitizer.php</exclude-pattern>
 	</rule>