C++: Always use the old library for the diff test

This change ensures that the diff test will show the difference between
the old and the new library even after we switch the default
implementation of `security.TaintTracking` to be the new one.

Author: jbj

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

@@ -1,6 +1,6 @@
 import cpp
 import semmle.code.cpp.security.Security
-import semmle.code.cpp.security.TaintTracking as ASTTaintTracking
+import semmle.code.cpp.security.TaintTrackingImpl as ASTTaintTracking
 import semmle.code.cpp.ir.dataflow.DefaultTaintTracking as IRDefaultTaintTracking
 
 predicate astFlow(Expr source, Element sink) { ASTTaintTracking::tainted(source, sink) }