Point to polyfils for running tests

Author: schlessera

features/install-wp-tests.feature

@@ -101,7 +101,13 @@ Feature: Scaffold install-wp-tests.sh tests
       wp_cli_test_scaffold
       """
 
-    When I run `WP_TESTS_DIR={RUN_DIR}/wordpress-tests-lib ./phpunit -c {PLUGIN_DIR}/hello-world/phpunit.xml.dist`
+    When I run `composer init --name=test/package --require="yoast/phpunit-polyfills:^1" --no-interaction --working-dir=polyfills`
+    Then the return code should be 0
+
+    When I run `composer install --no-interaction --working-dir=polyfills`
+    Then the return code should be 0
+
+    When I run `WP_TESTS_DIR={RUN_DIR}/wordpress-tests-lib WP_TESTS_PHPUNIT_POLYFILLS_PATH={RUN_DIR}/polyfills/vendor/yoast/phunit-polyfills ./phpunit -c {PLUGIN_DIR}/hello-world/phpunit.xml.dist`
     Then the return code should be 0
 
     When I try `WP_TESTS_DIR={RUN_DIR}/wordpress-tests-lib WP_CORE_DIR={RUN_DIR}/wordpress /usr/bin/env bash {PLUGIN_DIR}/hello-world/bin/install-wp-tests.sh wp_cli_test_scaffold {DB_USER} {DB_PASSWORD} {DB_HOST} latest < affirmative-response`