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

Created properties `SearchModel.BrowserCategoriesBuilder` and `SearchModel.AddonCategoriesBuilder`. They are used for test purposes.

#### Unit tests
Created new class `CategoryBuilderTests`. Moved there functions which are connected with categories manipulation.
![image](https://cloud.githubusercontent.com/assets/8158551/4921982/65c64f12-650e-11e4-8845-ac9ce311d5d8.png)

![image](https://cloud.githubusercontent.com/assets/8158551/4921989/7e007f94-650e-11e4-8986-d6eaf1b5a7fb.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.