Update unit test for the bug fix to abstract_environmentt

In a previous commit I fixed a bug where the default state
of a constructed abstract_environment was top rather than
bottom.  Correcting this meant this unit test fails as the
environment will only return objects which are bottom.

Also improve the tagging of the test.

Author: martin

unit/analyses/variable-sensitivity/eval.cpp

@@ -15,13 +15,16 @@ static symbolt simple_symbol(const irep_idt &identifier, const typet &type)
   return b1;
 }
 
-SCENARIO("eval", "[core]")
+SCENARIO(
+  "eval",
+  "[core][analyses][variable-sensitivity][interval_abstract_value]")
 {
   GIVEN("An environment with intervals domain")
   {
     variable_sensitivity_object_factoryt::instance().set_options(
       vsd_configt::intervals());
     abstract_environmentt environment;
+    environment.make_top(); // Domains are bottom on construction
 
     symbol_tablet symbol_table;
     namespacet ns{symbol_table};