Customize classpath when running tests

For some test scenarios (e.g. loading custom elements from the class path), it would be very convenient if one could manipulate the class path used to run the unit tests.

The attached patch allows a maven user to add a property

maven.test.classpath = foo.jar:bar.jar:baz.jar

to the project.properties and then the jar will be on the class path when the tests are run. This patch would help the Velocity project tremendously in running its unit tests, so please consider it for inclusion.