Add centralized test case with polyfills

tests/bootstrap.php

@@ -27,15 +27,9 @@
 		: '2.x.x'
 );
 
-/**
- * Compatibility with PHPUnit 6+
- */
-if ( class_exists( 'PHPUnit\Runner\Version' ) ) {
-	require_once __DIR__ . '/phpunit6-compat.php';
-}
-
 require_once VENDOR_DIR . '/autoload.php';
 require_once WP_CLI_ROOT . '/php/utils.php';
+require_once __DIR__ . '/wp-cli-testcase.php';
 
 function wpcli_tests_include_config( array $config_filenames = [] ) {
 	$config_filename = false;

tests/wp-cli-testcase.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace WP_CLI\Tests;
+
+use Yoast\PHPUnitPolyfills\TestCases\TestCase as PolyfilledTestCase;
+
+/**
+ * WP-CLI base test case.
+ */
+class TestCase extends PolyfilledTestCase {
+
+}