Fix return-type warnings with harness library and basic test (#776)

* Fix offending `-Wreturn-type` in harness library

* Fix offending `-Wreturn-type` in basic test

test_conformance/basic/test_progvar.cpp

@@ -599,6 +599,7 @@ static const TypeInfo& l_find_type( const char* name )
         if ( 0 == strcmp( name, type_info[i].get_name_c_str() ) ) return type_info[i];
     }
     assert(0);
+    return TypeInfo();
 }