diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 4839638..8572e92 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -15,8 +15,12 @@
     <testsuite name="unit">
       <directory>./tests/Drupal/Tests</directory>
       <directory>./modules/*/tests/src/Unit</directory>
-      <directory>../modules/*/tests/src/Unit</directory>
+			<!-- Allow for discovery of modules in sub-directories in the top-level
+			     modules/ directory -->
+      <directory>../modules</directory>
       <directory>../profiles/*/tests/src/Unit</directory>
+			<!-- Do not find tests in sub-directory modules under sites/. This is a
+			     security measure. Move modules to modules/ for testing. -->
       <directory>../sites/*/modules/*/tests/src/Unit</directory>
       <!-- Exclude Composer's vendor directory so we don't run tests there. -->
       <exclude>./vendor</exclude>