Package test-framework as individual jar

This commit causes all classes under 'org.elasticsearch.test.*'
to be included in a 'elasticsearch-${version}-test.jar' that can be
inclued by 3rd party projects or plugins via:

```
 <dependency>
    <groupId>org.elasticsearch</groupId>
    <artifactId>elasticsearch</artifactId>
    <version>${elasticsearch.version}</version>
    <type>test-jar</type>
    <scope>test</scope>
 </dependency>
```