python: dataflow for match

- also update `validTest.py`, but commented out for now
  otherwise CI will fail until we force it to run with Python 3.10
- added debug utility for dataflow (`dataflowTestPaths.ql`)

python/ql/test/experimental/dataflow/match/dataflowTest.ql

@@ -0,0 +1,13 @@
+import python
+import experimental.dataflow.TestUtil.FlowTest
+import experimental.dataflow.testConfig
+
+class DataFlowTest extends FlowTest {
+  DataFlowTest() { this = "DataFlowTest" }
+
+  override string flowTag() { result = "flow" }
+
+  override predicate relevantFlow(DataFlow::Node source, DataFlow::Node sink) {
+    exists(TestConfiguration cfg | cfg.hasFlow(source, sink))
+  }
+}