Python: Add tests of path-graph for dataflow/taint-tracking

Although this is actually using taint-tracking (so we can use the +=
statement), I would personally forget to check under the
dataflow/tainttracking folder to look for such a test, so I'm opting to
keep it under the dataflow/ folder.

python/ql/test/experimental/dataflow/path-graph/test.py

@@ -0,0 +1,96 @@
+def assign():
+    x = SOURCE # $ path-node
+
+    y = x
+
+    SINK(y) # $ path-node
+
+
+def aug_assign():
+    x = SOURCE # $ path-node
+    z = ""
+
+    z += x
+
+    SINK(z) # $ path-node
+
+
+def dont_use_rhs(cond):
+    # liked noted in the original Ruby PR: https://github.com/github/codeql/pull/12566
+    x = SOURCE # $ path-node
+
+    if cond:
+        y = x
+
+    SINK(x) # $ path-node
+
+
+def flow_through_function():
+    def identify(x): # $ path-node
+        return x # $ path-node
+
+    x = SOURCE # $ path-node
+
+    y = identify(x) # $ path-node
+
+    SINK(y) # $ path-node
+
+
+def attribute():
+    class X: pass
+    x = X()
+    x.attr = SOURCE # $ path-node
+
+    y = x
+
+    SINK(y.attr) # $ path-node
+
+
+def list_loop():
+    x = SOURCE # $ path-node
+    l = list()
+
+    l.append(x) # $ path-node
+
+    for y in l: # $ path-node
+
+        SINK(y) # $ path-node
+
+
+def list_index():
+    x = SOURCE # $ path-node
+    l = list()
+
+    l.append(x) # $ path-node
+
+    z = l[0] # $ path-node
+
+    SINK(z) # $ path-node
+
+
+def test_tuple():
+    x = SOURCE # $ path-node
+
+    y = ((x, 1), 2) # $ path-node
+
+    (z, _), _ = y # $ path-node
+
+    SINK(z) # $ path-node
+
+
+def test_with():
+    x = SOURCE # $ path-node
+
+    with x as y:
+
+        SINK(y) # $ path-node
+
+
+def test_match():
+    x = SOURCE # $ path-node
+
+    match x:
+
+        case y:
+
+            SINK(y) # $ path-node