Python: Add NeverReachable test

This looks for nodes annotated with `t.never` in the test that are
reachable in the CFG. This should not happen (it messes with various
queries, e.g. the "mixed returns" query), but the test shows that in a
few particular cases (involving the `match` statement where all cases
contain `return`s), we _do_ have reachable nodes that shouldn't be.

python/ql/test/library-tests/ControlFlow/evaluation-order/NeverReachable.expected

@@ -0,0 +1,2 @@
+| test_match.py:159:13:159:23 | BinaryExpr | Node annotated with t.never is reachable in $@ | test_match.py:151:1:151:42 | Function test_match_exhaustive_return_first | test_match_exhaustive_return_first |
+| test_match.py:172:13:172:23 | BinaryExpr | Node annotated with t.never is reachable in $@ | test_match.py:164:1:164:45 | Function test_match_exhaustive_return_wildcard | test_match_exhaustive_return_wildcard |

python/ql/test/library-tests/ControlFlow/evaluation-order/NeverReachable.ql

@@ -0,0 +1,26 @@
+/**
+ * Checks that expressions annotated with `t.never` either have no CFG
+ * node, or if they do, that the node is not reachable from its scope's
+ * entry (including within the same basic block).
+ */
+
+import python
+import TimerUtils
+
+from NeverTimerAnnotation ann
+where
+  exists(ControlFlowNode n, Scope s |
+    n.getNode() = ann.getExpr() and
+    s = n.getScope() and
+    (
+      // Reachable via inter-block path (includes same block)
+      s.getEntryNode().getBasicBlock().reaches(n.getBasicBlock())
+      or
+      // In same block as entry but at a later index
+      exists(BasicBlock bb, int i, int j |
+        bb.getNode(i) = s.getEntryNode() and bb.getNode(j) = n and i < j
+      )
+    )
+  )
+select ann, "Node annotated with t.never is reachable in $@", ann.getTestFunction(),
+  ann.getTestFunction().getName()