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.

Author: tausbn

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

@@ -0,0 +1,2 @@
+| test_match.py:159:13:159:13 | IntegerLiteral | 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:172:13:172:13 | IntegerLiteral | 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 +1,18 @@
+/**
+ * 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
+import OldCfgImpl
+
+private module Utils = EvalOrderCfgUtils<OldCfg>;
+
+private import Utils::CfgTests
+
+from TimerAnnotation ann
+where neverReachable(ann)
+select ann, "Node annotated with t.never is reachable in $@", ann.getTestFunction(),
+  ann.getTestFunction().getName()