Create unittests for ast

There was some code in `unittest{}` section, but it
only printed string version on terminal
and one had to manualy check if result was as expected

Now checks are done automaticaly
Specify import in more detailed way

Instead of 'import package', use 'import package:exactName' syntax
Flaten resulting Sequence ast

Instead of Seq[Seq[L(a),L(b)],Seq[L(c),L(d)]]
now make equivalent but simpler Seq[L(a),L(b),L(c),L(d)]