Fix shadow memory var-assign1 regression test

Author: Enrico Steffinlongo

regression/cbmc-shadow-memory/var-assign1/main.c

@@ -2,9 +2,9 @@
 
 int main()
 {
-  __CPROVER_field_decl_local("field", (_Bool)0);
+  __CPROVER_field_decl_local("field", (char)0);
 
-  _Bool z;
+  char z;
   int x;
   __CPROVER_set_field(&x, "field", z);
   assert(__CPROVER_get_field(&x, "field") == z);

regression/cbmc-shadow-memory/var-assign1/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 
 ^EXIT=0$