Ensure DrupalorgVersioncontrolLabelVersionMapperGit.class.php is working properly in D7

As part of #2137201: [META] Regressions in project_release handling of version element fields and release-history XML we're going to start relying on the proper functioning of the label_version_mapper plugin from versioncontrol_release. The d.o implementation of this plugin was ported to D7, but it's unclear if it's really working right, since the code that invokes it isn't being invoked the same way it was in D6.

So, once #2145667: project_release_node_presave() should only parse the version string if the version elements are not already set and #2145675: Alter the release node form to set values for the version element fields as determined by the label_version_mapper plugin are done, we need to test this plugin and make sure it's doing what it's supposed to.