libtiled-java: Add required components for unit-tests

* Run "ant test" to run all unit tests
* Unit-test source files are kept in "test/", separate from the regular "src/" source files.
* When building, unit test compilation output is also separated, since we want the unit test compilation to use "debug=true".

This PR only includes rudimentary unit-tests, to verify that the unit-test harness works. Will add more actual unit-tests later.