Python: Expand type-tracking tests with nested tuples

I was initially surprised to see that this didn't work, until I
remembered that type-tracking only works with content of depth 1.

Author: RasmusWL

python/ql/test/experimental/dataflow/typetracking/CONSISTENCY/TypeTrackingConsistency.expected

@@ -0,0 +1,13 @@
+unreachableNode
+| content_test.py:31:6:31:11 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 0. |
+| content_test.py:31:6:31:11 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 1. |
+| content_test.py:31:6:31:11 | ControlFlowNode for Tuple | Unreachable node in step of kind storeTarget. |
+| content_test.py:31:16:31:21 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 0. |
+| content_test.py:31:16:31:21 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 1. |
+| content_test.py:31:16:31:21 | ControlFlowNode for Tuple | Unreachable node in step of kind storeTarget. |
+| content_test.py:40:10:40:13 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 0. |
+| content_test.py:40:10:40:13 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 1. |
+| content_test.py:40:10:40:13 | ControlFlowNode for Tuple | Unreachable node in step of kind storeTarget. |
+| content_test.py:66:9:66:12 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 0. |
+| content_test.py:66:9:66:12 | ControlFlowNode for Tuple | Unreachable node in step of kind load Tuple element at index 1. |
+| content_test.py:66:9:66:12 | ControlFlowNode for Tuple | Unreachable node in step of kind storeTarget. |

python/ql/test/experimental/dataflow/typetracking/content_test.py

@@ -21,6 +21,28 @@ def test_tuple(index_arg):
         print(tup[i])
 
 
+    # nested tuples
+    nested_tuples = ((tracked, other), (other, tracked)) # $tracked
+
+    nested_tuples[0][0] # $ MISSING: tracked
+    nested_tuples[0][1]
+    nested_tuples[1][0]
+    nested_tuples[1][1] # $ MISSING: tracked
+
+    (aa, ab), (ba, bb) = nested_tuples
+    aa # $ MISSING: tracked
+    ab
+    ba
+    bb # $ MISSING: tracked
+
+
+    # non-precise access is not supported right now (and it's not 100% clear if we want
+    # to support it, or if it will lead to bad results)
+    for (x, y) in nested_tuples:
+        x
+        y
+
+
 def test_dict(key_arg):
     d1 = {"t": tracked, "o": other} # $tracked
     d1["t"] # $ tracked