Using it in tests

[dcalap]

Add some tests.

Regarding this problem I had -> https://github.com/tbroyer/gradle-apt-plugin/issues/73

Just notice that if you try to use this combo in tests, you need to use testApt in the build.gradle dependencies instead of apt.

[filiphr]

Thanks for the info @dcalap. In case you have time you can add such examples to the mapstruct-on-gradle example