Assert: Fix assertions called directly from QUnit.assert

Found this bug on running the jQuery Core tests.

This is a backwards compatibility fix.
we need to have the current test as the context for the `test.push` to prevent async leaking. 

```js
return self.test.push.apply( self.test, arguments );
                                ^^^
```
When using directly assertions without assertion context, like: `equal` and `QUnit.assert.equal`, we lose the test reference in this method.