Extracted add function.

I really don't know is it feature or not. Assertions get doubled if I run Microtest::run() multiple times. If you use same tests against different external dependencies it can be cool. But it's become too many assertions in results table and when they use different external dependencies which I'm not expect and they fail. So now:
```php
    Microtest::add(...);
    Microtest::add(...);
    ...
    Microtest::run();
    Microtest::resultsHTML();
```
And of course  ```php Microtest::run(array(function (){})); ``` also works.