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/lib/utils/test/dataflow/MaximalFlowTest.qll

@@ -35,7 +35,7 @@ module MaximalFlowsConfig implements DataFlow::ConfigSig {
   predicate isSink(DataFlow::Node node) {
     exists(node.getLocation().getFile().getRelativePath()) and
     not any(CallNode c).getArg(_) = node.asCfgNode() and
-    not node instanceof DataFlow::ArgumentNode and
+    not isArgumentNode(node, _, _) and
     not node.asCfgNode().(NameNode).getId().matches("SINK%") and
     not DataFlow::localFlowStep(node, _)
   }

python/ql/lib/utils/test/dataflow/callGraphConfig.qll

@@ -9,7 +9,7 @@ module CallGraphConfig implements DataFlow::ConfigSig {
   predicate isSource(DataFlow::Node node) {
     node instanceof DataFlowPrivate::ReturnNode
     or
-    node instanceof DataFlow::ArgumentNode
+    DataFlowPrivate::isArgumentNode(node, _, _)
   }
 
   predicate isSink(DataFlow::Node node) {