Make functions non-empty so that the test remains stable under skip cleanup

Author: tautschnig

jbmc/regression/jbmc/virtual8/A.class

@@ -10,16 +10,16 @@ public static void main() {
 }
 
 class A {
-  void f() { }
+  int f() { return 0; }
 }
 
 
 class B extends A {
-  void f() { }
+  int f() { return 0; }
 }
 
 class C extends A {
-  void f() { }
+  int f() { return 0; }
 }
 
 class D extends C {