Feature/run all tests with same backend

Enables picking a backend to run all tests, e.g.:

    mvn test -Dcode.generator.class=pt.ist.fenixframework.backend.jvstm.JVSTMCodeGenerator

Also, we can select tests to exclude, e.g.:

    mvn test -Dcode.generator.class=pt.ist.fenixframework.backend.jvstm.JVSTMCodeGenerator -Dtest.excludes="**/DmlSlotsTest.java,**/NestingTest.java" -pl test -amd