C++: Don't use C-style varargs in test.cpp sink

As we prepare to clarify how conversions are treated, we don't want a
`sink(...)` declaration where it's non-obvious which conversions are
applied to arguments.

cpp/ql/test/library-tests/dataflow/dataflow-tests/test.cpp

@@ -1,5 +1,5 @@
 int source();
-void sink(...);
+void sink(int); void sink(const int *); void sink(int **);
 
 void intraprocedural_with_local_flow() {
   int t2;