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>
```

pom.xml

@@ -975,6 +975,23 @@
                     </arguments>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-jar-plugin</artifactId>
+                <version>2.3.2</version>
+                <executions>
+                    <execution>
+                        <goals>
+                            <goal>test-jar</goal>
+                        </goals>
+                    </execution>
+                </executions>
+                <configuration>
+                    <includes>
+                        <include>org/elasticsearch/test/**/*</include>
+                    </includes>
+                </configuration>
+            </plugin>
         </plugins>
         <pluginManagement>
             <plugins>