Fix printing of smt_get_value_commandt

The get value command is supposed to be parameterised with a collection
of one or more terms for which the solver should return the values. Our
internal representation of the get-value command only supports a single
term, which should be placed in brackets to make it into a collection
containing a single term. The brackets were previously missing, which
would have caused a solver error.

Author: thomasspriggs

src/solvers/smt2_incremental/smt_to_smt2_string.cpp

@@ -334,7 +334,7 @@ class smt_command_to_string_convertert
 
   void visit(const smt_get_value_commandt &get_value) override
   {
-    os << "(get-value " << get_value.descriptor() << ")";
+    os << "(get-value (" << get_value.descriptor() << "))";
   }
 
   void visit(const smt_pop_commandt &pop) override

unit/solvers/smt2_incremental/smt_to_smt2_string.cpp

@@ -75,7 +75,7 @@ TEST_CASE(
 {
   CHECK(
     smt_to_smt2_string(smt_get_value_commandt{
-      smt_identifier_termt{"foo", smt_bool_sortt{}}}) == "(get-value |foo|)");
+      smt_identifier_termt{"foo", smt_bool_sortt{}}}) == "(get-value (|foo|))");
 }
 
 TEST_CASE(