Rewrite unit tests for categories manipulation

#### Purpose

Earlier in this PR https://github.com/Benglin/Dynamo/pull/132 was made separation code to `SearchModel` and new class `CategoryBuilder`. Tests which are use some functions of `SearchModel` were skipped. This PR adapts `SearchModelTests` to new reality.

#### Modifications

`SearchModel.browserCategoriesBuilder` and `SearchModel.addonCategoriesBuilder` are made as internal to have access from `DynamoCoreTests` project. In `SearcModelTests` used `search.addonCategoriesBuilder` instead of direct `SearchModel` functions. For all cases except one is used addons `CategoryBuilder`.

#### Unit tests
![image](https://cloud.githubusercontent.com/assets/8158551/4904201/5d713188-6446-11e4-9daf-a55e91f9c163.png)

#### Additional references 

[MAGN-5157](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5157).

Next planned is adding tests to test differences in work of `CategoryBuilder` for regular nodes and addons. Task:  [MAGN-5304](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5304).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
It seems like (from this test case and those updated below) we are no longer testing `SearchModel` class. These tests are testing `CategoryBuilder` which is good, but they are not testing the integration of `CategoryBuilder` with `SearchModel`.

When `CategoryBuilder` was first introduced, you've updated quite a bit of code in `SearchModel`. It is important to also test those integration parts. For example, now we have `addonCatBuilder.AddCategory` tested here, wouldn't it be nicer if we also test the following code (by testing `TryAddCategoryAndItem`) you have added?

```cs
internal void TryAddCategoryAndItem(string category, BrowserInternalElement item)
{
    ElementType nodeType = GetElementType(item);

    BrowserItem cat;
    if (nodeType == ElementType.Regular)
        cat = browserCategoriesBuilder.AddCategory(category, (item as NodeSearchElement).Assembly);
    else
        cat = addonCategoriesBuilder.AddCategory(category, (item as NodeSearchElement).Assembly);

    // Other codes...
}
```