I introduced bug with last counterexample commit. This was due to
querying counterexample value of variables that were not introduced. This
commits introduce a container that save introduced variables and check if
they should be taken as counterex.

* src/transform/intro_vc_vars_counterexmp.ml
(do_intro): Adding vc_var which contains all variables that we can safely
print as counterexamples.
(remove_positive_foralls): Adding vc_var and introduce variables in vc_var
when quantified over.
(intros): Adding vc_var.
(do_intro_vc_vars_counterexmp): Adding vc_var.

Change-Id: Ic6bf732f1e50241a42df8e097f52aa46dd473bd2