java: add tests for java code

The tests can be run on the host, using host's JRE like this:

  cd java; mvn test

This is mainly useful during development to have a short modify-test
cycle. The tests should be eventually executed on OSv and support for
that will follow.

The test code is split into two modules which yield two separate jars:

 tests          - contains test cases
 tests-isolates - contains code which is started in isolated context

We need separate jars for these two to have different classpath in the
isolate and thus more realistic tests.