Download junit4-stub JAR file with all dependencies

Stub used instead of JUnit 4 dependency. These classes are used only so that the class hiearchy can be maintained. Functionality of these classes is not used, that's why they're empty, but they're needed to successfuly compile 3rd party dependencies without introducing junit:junit (JUnit 4) dependency. For one such case see: https://github.com/testcontainers/testcontainers-java/issues/970#issuecomment-625044008