diff --git a/composer.json b/composer.json
index 3aa4c28ad..39e83cbd0 100644
--- a/composer.json
+++ b/composer.json
@@ -20,11 +20,11 @@
         "wp-cli/core-command": "^1 || ^2",
         "wp-cli/eval-command": "^1 || ^2",
         "wp-cli/wp-cli": "^2",
-        "wp-coding-standards/wpcs": "^2.1"
+        "wp-coding-standards/wpcs": "^2.1",
+        "yoast/phpunit-polyfills": "^0.2.0"
     },
     "require-dev": {
-        "roave/security-advisories": "dev-master",
-        "yoast/phpunit-polyfills": "^0.2.0"
+        "roave/security-advisories": "dev-master"
     },
     "config": {
         "platform": {
diff --git a/tests/phpunit6-compat.php b/tests/phpunit6-compat.php
deleted file mode 100644
index 4e57f5fd4..000000000
--- a/tests/phpunit6-compat.php
+++ /dev/null
@@ -1,19 +0,0 @@
-<?php
-// From core "tests/phpunit/includes/phpunit6-compat.php" without `getTickets()` (see https://core.trac.wordpress.org/ticket/39822).
-
-if ( class_exists( 'PHPUnit\Runner\Version' ) && version_compare( PHPUnit\Runner\Version::id(), '6.0', '>=' ) ) {
-
-	class_alias( 'PHPUnit\Framework\TestCase', 'PHPUnit_Framework_TestCase' );
-	class_alias( 'PHPUnit\Framework\Exception', 'PHPUnit_Framework_Exception' );
-	class_alias( 'PHPUnit\Framework\ExpectationFailedException', 'PHPUnit_Framework_ExpectationFailedException' );
-	class_alias( 'PHPUnit\Framework\Error\Notice', 'PHPUnit_Framework_Error_Notice' );
-	class_alias( 'PHPUnit\Framework\Error\Warning', 'PHPUnit_Framework_Error_Warning' );
-	class_alias( 'PHPUnit\Framework\Test', 'PHPUnit_Framework_Test' );
-	class_alias( 'PHPUnit\Framework\Warning', 'PHPUnit_Framework_Warning' );
-	class_alias( 'PHPUnit\Framework\AssertionFailedError', 'PHPUnit_Framework_AssertionFailedError' );
-	class_alias( 'PHPUnit\Framework\TestSuite', 'PHPUnit_Framework_TestSuite' );
-	class_alias( 'PHPUnit\Framework\TestListener', 'PHPUnit_Framework_TestListener' );
-	class_alias( 'PHPUnit\Util\GlobalState', 'PHPUnit_Util_GlobalState' );
-	class_alias( 'PHPUnit\Util\Getopt', 'PHPUnit_Util_Getopt' );
-
-}