diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index bd0edc9e86cb768bdc3d385c0468b29fa2cf2baa..cf086006eace6e08d65d98129f54fcb932a655a1 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -33,6 +33,11 @@
       <directory>./modules</directory>
       <directory>../modules</directory>
       <directory>../sites</directory>
+      <!-- By definition test classes have no tests. -->
+      <exclude>
+        <directory suffix="Test.php">./</directory>
+        <directory suffix="TestBase.php">./</directory>
+      </exclude>
      </whitelist>
   </filter>
 </phpunit>