Python: Fix test issues

Fixes the test failures that arose from making `ExtractedArgumentNode`
local.

For the consistency checks, we now explicitly exclude the
`ExtractedArgumentNode`s (now much more plentiful due to the
overapproximation) that don't have a corresponding `getCallArg` tuple.

For various queries/tests using `instanceof ArgumentNode`, we instead us
`isArgumentNode`, which explicitly filters out the ones for which
`isArgumentOf` doesn't hold (which, again, is the case for most of the
nodes in the overapproximation).

python/ql/src/Classes/InitCallsSubclass/InitCallsSubclassMethod.ql

@@ -15,6 +15,7 @@
 import python
 import semmle.python.dataflow.new.DataFlow
 import semmle.python.dataflow.new.internal.DataFlowDispatch
+import semmle.python.dataflow.new.internal.DataFlowPrivate
 
 predicate initSelfCallOverridden(
   Function init, DataFlow::Node self, DataFlow::MethodCallNode call, Function target,
@@ -39,7 +40,7 @@ predicate readsFromSelf(Function method) {
     self.getParameter() = method.getArg(0) and
     DataFlow::localFlow(self, sink)
   |
-    sink instanceof DataFlow::ArgumentNode
+    isArgumentNode(sink, _, _)
     or
     sink = any(DataFlow::AttrRead a).getObject()
   )