Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
No they couldn't be reused because we don't know what a user is doing in their tag helper.  Ex:

```C#
class FooTagHelper : TagHelper
{
    private string _something;

    public void Process(...)
    {
        if(output.HasSomething...)
        {
            _something = "Hello World";
        }
        else
        {
            _something = "somethingdifferent";
        }
    }
}
```
We wouldn't know how to reset the `_something`.