Correctly set up the enviroment before running tests

In release builds, the top/bottom status of the enviroment was
undefined. This ensures the enviroment is set to top (i.e. an entry
point).

Author: thk123

unit/analyses/variable-sensitivity/constant_abstract_value/merge.cpp

@@ -32,6 +32,7 @@ SCENARIO("merge_constant_abstract_value",
     const exprt val2=constant_exprt::integer_constant(2);
 
     abstract_environmentt enviroment;
+    enviroment.make_top();
     symbol_tablet symbol_table;
     namespacet ns(symbol_table);
 

unit/analyses/variable-sensitivity/constant_array_abstract_object/merge.cpp

@@ -97,6 +97,7 @@ SCENARIO("merge_constant_array_abstract_object",
       index_exprt(nil_exprt(), constant_exprt::integer_constant(2));
 
     abstract_environmentt enviroment;
+    enviroment.make_top();
     symbol_tablet symbol_table;
     namespacet ns(symbol_table);
 

unit/analyses/variable-sensitivity/full_struct_abstract_object/merge.cpp

@@ -128,6 +128,7 @@ SCENARIO("merge_full_struct_abstract_object",
     const member_exprt c(nil_exprt(), "c");
 
     abstract_environmentt enviroment;
+    enviroment.make_top();
     symbol_tablet symbol_table;
     namespacet ns(symbol_table);