Switch from deprecated QUnit.load() to QUnit.start()

QUnit.load has been superseded by QUnit.start in QUnit 2.1,
and QUnit.load emits deprecation warnings since QUnit 2.21.

This fixes the following warning:

> QUnit.load is deprecated and will be removed in QUnit 3.0.
> https://qunitjs.com/api/QUnit/load/

Author: Krinkle

lib/child.js

@@ -162,4 +162,4 @@ options.tests.forEach(function(test) {
     _require(test, false);
 });
 
-QUnit.load();
+QUnit.start();