Python: Add a few test-cases for barrier guards and references

I'm not sure references is the best name, but it's the best I could come up with
jsut now

python/ql/test/experimental/dataflow/tainttracking/customSanitizer/TestTaint.expected

@@ -34,6 +34,14 @@ test_taint
 | test_logical.py:128 | ok   | test_nesting_not_with_and_true | s |
 | test_logical.py:137 | fail | test_with_return | s |
 | test_logical.py:146 | fail | test_with_exception | s |
+| test_reference.py:31 | fail | test_basic | s2 |
+| test_reference.py:31 | ok   | test_basic | s |
+| test_reference.py:33 | ok   | test_basic | s |
+| test_reference.py:33 | ok   | test_basic | s2 |
+| test_reference.py:41 | fail | test_identical_call | s.strip() |
+| test_reference.py:43 | ok   | test_identical_call | s.strip() |
+| test_reference.py:56 | fail | test_class_attribute_access | c.foo |
+| test_reference.py:58 | ok   | test_class_attribute_access | c.foo |
 isSanitizer
 | TestTaintTrackingConfiguration | test.py:21:39:21:39 | ControlFlowNode for s |
 | TestTaintTrackingConfiguration | test.py:50:10:50:29 | ControlFlowNode for emulated_escaping() |
@@ -48,3 +56,6 @@ isSanitizerGuard
 | TestTaintTrackingConfiguration | test_logical.py:115:12:115:21 | ControlFlowNode for is_safe() |
 | TestTaintTrackingConfiguration | test_logical.py:120:16:120:25 | ControlFlowNode for is_safe() |
 | TestTaintTrackingConfiguration | test_logical.py:125:20:125:29 | ControlFlowNode for is_safe() |
+| TestTaintTrackingConfiguration | test_reference.py:30:8:30:17 | ControlFlowNode for is_safe() |
+| TestTaintTrackingConfiguration | test_reference.py:40:8:40:25 | ControlFlowNode for is_safe() |
+| TestTaintTrackingConfiguration | test_reference.py:55:8:55:21 | ControlFlowNode for is_safe() |

python/ql/test/experimental/dataflow/tainttracking/customSanitizer/test_reference.py

@@ -0,0 +1,65 @@
+import sys; import os; sys.path.append(os.path.dirname(os.path.dirname((__file__))))
+from taintlib import *
+
+# This has no runtime impact, but allows autocomplete to work
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from ..taintlib import *
+
+
+# Actual tests
+
+"""Testing logical constructs not/and/or works out of the box.
+"""
+
+import random
+
+
+def random_choice():
+    return bool(random.randint(0, 1))
+
+
+def is_safe(arg):
+    return arg == "safe"
+
+
+def test_basic():
+    s = TAINTED_STRING
+    s2 = s
+
+    if is_safe(s):
+        ensure_not_tainted(s, s2)
+    else:
+        ensure_tainted(s, s2)
+
+
+def test_identical_call():
+    """This code pattern is being used in real world code"""
+    s = TAINTED_STRING
+
+    if is_safe(s.strip()):
+        ensure_not_tainted(s.strip())
+    else:
+        ensure_tainted(s.strip())
+
+
+class C(object):
+    def __init__(self, value):
+        self.foo = value
+
+
+def test_class_attribute_access():
+    s = TAINTED_STRING
+    c = C(s)
+
+    if is_safe(c.foo):
+        ensure_not_tainted(c.foo)
+    else:
+        ensure_tainted(c.foo)
+
+
+# Make tests runable
+
+test_basic()
+test_identical_call()
+test_class_attribute_access()