Don't overwrite an existing wp-tests-config.php (#1003)

Author: akirk

bin/install-wp-tests.sh

@@ -111,7 +111,7 @@ install_test_suite() {
 		svn co --quiet --ignore-externals https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/data/ $WP_TESTS_DIR/data
 	fi
 
-	if [ ! -f wp-tests-config.php ]; then
+	if [ ! -f "$WP_TESTS_DIR"/wp-tests-config.php ]; then
 		download https://develop.svn.wordpress.org/${WP_TESTS_TAG}/wp-tests-config-sample.php "$WP_TESTS_DIR"/wp-tests-config.php
 		# remove all forward slashes in the end
 		WP_CORE_DIR=$(echo $WP_CORE_DIR | sed "s:/\+$::")