Fix PHPUnit coverage configuration to use 'classes' directory

The coverage configuration was pointing to a non-existent 'src' directory, causing PHPUnit to not generate any coverage data. Changed to 'classes' which is the actual source code directory.

This was the root cause of 0% coverage reporting.

Author: aristath

phpunit.xml.dist

@@ -14,7 +14,7 @@
 	</testsuites>
 	<coverage>
     <include>
-        <directory suffix=".php">src</directory>
+        <directory suffix=".php">classes</directory>
 		<file>progress-planner.php</file>
     </include>
   </coverage>