tools/jpackage tests do not work on Ubuntu Linux

Currently the tools/jpackage tests do not work on Ubuntu Linux.
Seems this is expected, a question on corelibs-dev was answered :

> > Do you expect those tests to work on Ubuntu ?
> No, rpm test are not supposed to pass on Ubuntu.
> To disable running rpm tests on Ubuntu please set
> "jpackage.test.disabledPackagers" system property to "rpm":
> -Djpackage.test.disabledPackagers=rpm.

So the tests should find out that they are running on Ubuntu, and in this case the rpm packager should be disabled.