MDL-66903 testing: Add support for a \tests\ namespace during tests

This commit:
- introduces a \tests\ sub-namespace for use in unit tests only
- the path to this the tests/classes directory of the owning parent
- files here are excluded from unit test runs

This is agreed per policy in MDL-80855.

lib/phpunit/classes/util.php

@@ -529,6 +529,7 @@ class phpunit_util extends testing_util {
         $template = <<<EOF
             <testsuite name="@component@_testsuite">
               <directory suffix="_test.php">@dir@</directory>
+              <exclude>@dir@/classes</exclude>
             </testsuite>
 
         EOF;
@@ -621,6 +622,7 @@ class phpunit_util extends testing_util {
             <testsuites>
               <testsuite name="@component@_testsuite">
                 <directory suffix="_test.php">.</directory>
+                <exclude>./classes</exclude>
               </testsuite>
             </testsuites>
           EOT;