BUG: Asserting after `test.done()` is allowed

``` javascript
exports.test1 = function(test) {
test.expect(1);
test.ok(true, 'is true');
test.done();

// this should throw an error, however it passes
test.ok(true, 'is true2');
};
```

Start by reproducing the example using test.ok after test.done and trace how those entry points handle completion and later assertions. The work is done when an assertion after test.done throws the expected error while assertions before completion retain their current behavior, with the regression covered by the project's tests.

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