Python: Add example of missing use-use flow

(see PR for more detailed description)

Author: RasmusWL

python/ql/test/library-tests/essa/ssa-compute/test2.py

@@ -0,0 +1,29 @@
+def func(x): # $ def=x
+    try:
+        with Thing() as y: # $ def=y
+            y.foo(x, 0) # $ def-use=x:1 def-use=y:3
+            while not x.attribute: # $ use-use=x:4 use-use=x:7
+                y.bar() # $ use-use=y:4 use-use=y:6
+                print(x) # $ use-use=x:5
+    finally:
+        pass
+
+def func(x): # $ def=x
+    try:
+        with Thing() as y: # $ def=y
+            y.foo(x, some_var) # $ def-use=x:11 def-use=y:13
+            while not x.attribute: # $ use-use=x:14 use-use=x:17
+                y.bar() # $ use-use=y:16 MISSING: use-use=y:14
+                print(x) # $ use-use=x:15
+    finally:
+        pass
+
+def func(x): # $ def=x
+    try:
+        with Thing() as y: # $ def=y
+            y.foo(x, some_var.some_attr) # $ def-use=x:21 def-use=y:23
+            while not x.attribute: # $ use-use=x:27 MISSING: use-use=x:24
+                y.bar() # $ use-use=y:26 MISSING: use-use=y:24
+                print(x) # $ use-use=x:25
+    finally:
+        pass