Remove `JSON_UNESCAPED_SLASHES` from the `wp_json_encode` call - this constant is not supported in PHP < 5.4, and we don't polyfill it either.
Also make the PHPUnit test suite correctly exit with a non-zero exit code when `wp-tests-config.php` is not present. This was causing `grunt restapi-jsclient` to incorrectly proceed to its second step even when the first step failed with this error.
Props ocean90.
Fixes#39264.
git-svn-id: https://develop.svn.wordpress.org/trunk@40065 602fd350-edb4-49c9-b593-d223f7449a82