diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 95883b1..3ca3a3a 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -47,6 +47,15 @@ static function ( $fqClassName ) {
 	die( 1 );
 }
 
+/*
+ * Define the constant because our tests are running PHPUnit test cases themselves.
+ * This will prevent some tests being marked as "risky" on old PHPUnit versions for
+ * closing buffers.
+ */
+if ( \defined( 'PHPUNIT_TESTSUITE' ) === false ) {
+	\define( 'PHPUNIT_TESTSUITE', true );
+}
+
 /*
  * Create a number of class aliases for PHPUnit native classes which have been
  * renamed over time and are only used in the unit tests.