From these docs: http://www.eclipse.org/jetty/documentation/current/jetty-maven-plugin.html#jetty-run-goal
I think you just need to configure the contextPath in your pom.xml:
<plugin>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-maven-plugin</artifactId>
<configuration>
<scanIntervalSeconds>10</scanIntervalSeconds>
<webApp>
<contextPath>/test</contextPath>
</webApp>
</configuration>
</plugin>