Improve tests + new tests

...in/java/be/cylab/java/wowa/training/AbstractSolution.java

src/main/java/be/cylab/java/wowa/training/Example.java

src/main/java/be/cylab/java/wowa/training/Utils.java

src/main/resources/data_test.json

src/main/resources/expected_test.json

src/test/java/be/cylab/java/wowa/training/UtilsTest.java