Convert tests to JUnit

In preparation to port gstreamer-java tests, and for general
advancement over the "quick and dirty" test runner, convert tests,
and the corresponding build targets to JUnit.
Add missing -1 to QSortTest

Left out by mistake, present in the original C version.
Fix copy-paste error in Singleton test
Redefine constructor in MySingletonClass instead of parent class

This is the correct thing to do, and also what the original C
version does. This fixes the Singleton test.