Added unit tests to test uncovered places of CategoryBuilder / SearchModel interaction

#### Purpose

Add unit tests to test uncovered places of CategoryBuilder / SearchModel interaction.

#### Unit tests

![image](https://cloud.githubusercontent.com/assets/8158551/5079407/c7ec9c5c-6ebc-11e4-8523-f2800464e2c2.png)
![image](https://cloud.githubusercontent.com/assets/8158551/5079415/e3950462-6ebc-11e4-9361-19233ab49851.png)

#### Additional references

[MAGN-5304](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5304).

#### Reviewers

@Benglin, please, take a look.