Python: Add test for type-tracking through decorators

In general, if there is _some_ decorator on a function, it might not be safe to
track content out of it (since the decorator could do anything), but in this
case, we can see what the decorator does, so we should be able to handle it (but
we don't right now).

By my understanding of how type-tracking works, if we track content through
`my_decorator`, then we would also track content to the result of
`unrelated_func()`, which I wanted to make sure our tests would catch.

I found out the core of the problem seems to come from our lack of being able to
track to the inner scope, and added an explicit test for that.

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

@@ -56,6 +56,41 @@ def test_import():
     y # $tracked
     mymodule.z # $tracked
 
+
+def to_inner_scope():
+    x = tracked # $tracked
+    def foo():
+        y = x # $ MISSING: tracked
+        return y # $ MISSING: tracked
+    also_x = foo() # $ MISSING: tracked
+    print(also_x) # $ MISSING: tracked
+
+
+def my_decorator(func):
+    # This part doesn't make any sense in a normal decorator, but just shows how we
+    # handle type-tracking
+
+    func() # $tracked
+
+    def wrapper():
+        print("before function call")
+        val = func() # $ MISSING: tracked
+        print("after function call")
+        return val # $ MISSING: tracked
+    return wrapper
+
+@my_decorator
+def get_tracked2():
+    return tracked # $tracked
+
+@my_decorator
+def unrelated_func():
+    return "foo"
+
+def use_funcs_with_decorators():
+    x = get_tracked2() # $ MISSING: tracked
+    y = unrelated_func()
+
 # ------------------------------------------------------------------------------
 
 def expects_int(x): # $int