[SCM-786] Add missing dependency on junit to maven-scm-test

`maven-scm-test` uses JUnit, but doesn't declare it as dependency. This works because JUnit is a transitive dependency pulled in by Plexus container. However in Plexus 1.6 scope of JUnit was changed to "provided" meaning that maven-scm-test won't work with Plexus 1.6 unless explicit dependency on JUnit is added.