test.py: run java tests

Java tests did not fit into current model well. I created a basic
abstration which allows to fit java tests here.

The --single mode was especially problematic because it assumed that
testrunner.so is able to run all the tests. This is no longer the
case. I changed it so that all tests managed by testrunner are ran in
a single guest instance and all other tests are ran one-by-one.