It should be possible to add additional elements to the test classpath

(this is basically a rehash of MPTEST-16)

For porting unit tests from ant to maven, it was necessary to add certain pre-produced jars to the test classpath. This is not possible with the 1.6.2 plugin. My solution was (as the solution of some other users), to add a property which can
be assigned and is then added to the classpath. This allows me to do e.g.

maven.test.classpath= $

/test/cpload/test2.jar

where test1.jar and test2.jar are jars that must not be present in the build phase but need to present in the test phase.

There have different solution been discussed. However:

• adding something to the "resources" part of the POM makes it impossible to automatically download such a jar
  through maven.
• adding a property to the dependencies makes it impossible to ship such a test jar with the distribution and reference
  it locally.

and still we currently have no solution at all. I'd therefore propose the attached patch which allows all three test goals in maven-test-plugin (test:single, test:test and test:match) to evaluate a property variable called maven.test.classpath. This
variable (if nonempty) is transferred to a <pathelement path="..."/> element and can therefore contain multiple elements (separated by e.g. ";").

The attached patch is IMHO the easiest fix for this problem with seems to affect a lot of maven users.