Allow to configure JVM for tests by referencing a toolchain entry

There is the property "jvm" which allows to specify a specific JVM to run on the tests through Surefire. This puts in an absolute path into my POM, though, which makes the build less portable.

Therefore I propose to allow an alternative option which allows to refer to specific JDKs configured through toolchains.xml:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <toolchain>
            <jdk>
                <version>1.7</version>
                <vendor>sun</vendor>
            </jdk>
        </toolchains> 
    </configuration>
</plugin>

Note that this allows to use a specific toolchain just for tests, independently what's configured as toolchain for the overall build through the maven-toolchain-plugin. This e.g. allows to run the build on JDK 8 but run tests on JDK/JRE 7 (which is a frequent requirement for our projects). Or one could have several Surefire executions, running tests on different Java versions.

The toolchain to use should be injectable through a property, so one can easily configure it, e.g. for specific Jenkins runs: "... -Dsurefire.toolchain=jdk:1.7" or "... -Dsurefire.toolchain=jdk:1.7:sun".