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/PathNodes.expected

@@ -0,0 +1,2 @@
+failures
+testFailures

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

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+/**
+ * @kind path-problem
+ */
+
+import python
+import semmle.python.dataflow.new.DataFlow
+import semmle.python.dataflow.new.TaintTracking
+import experimental.dataflow.testConfig
+import TestUtilities.InlineExpectationsTest
+
+module TestTaintFlow = TaintTracking::Global<TestConfig>;
+
+module PathNodeTest implements TestSig {
+  string getARelevantTag() { result = "path-node" }
+
+  predicate hasActualResult(Location location, string element, string tag, string value) {
+    exists(TestTaintFlow::PathNode pn |
+      location = pn.getNode().getLocation() and
+      tag = "path-node" and
+      value = "" and
+      element = pn.toString()
+    )
+  }
+}
+
+import MakeTest<PathNodeTest>
+// running the query to inspect the results can be quite nice!
+// just uncomment these lines!
+// import TestTaintFlow::PathGraph
+// from TestTaintFlow::PathNode source, TestTaintFlow::PathNode sink
+// where TestTaintFlow::flowPath(source, sink)
+// select sink.getNode(), source, sink,
+//   sink.getNode().getEnclosingCallable().toString() + ": --> " +
+//     sink.getNode().getLocation().getStartLine().toString()

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

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+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