Download all versions of taverna-reference-testhelpers JAR files with all dependencies

In order to properly test OSGi-loaded t2ference modules we need a module that is entire external to t2reference and to the test cases. If the test implementations are included in either the api, core implementations or test modules they will be loaded by the root classloader of the test runner - by putting them in an independent artifact we allow them to be loaded through various SPI discovery mechanisms as they would be in a 'real' environment.