add test

tests/ast/test_metadata_journal.py

@@ -0,0 +1,21 @@
+from vyper.ast.metadata import NodeMetadata
+from vyper.exceptions import VyperException
+
+
+def test_metadata_journal():
+    m = NodeMetadata()
+
+    m["x"] = 1
+    try:
+        with m.enter_typechecker_speculation():
+            m["x"] = 2
+            m["x"] = 3
+
+            assert m["x"] == 3
+            raise VyperException("dummy exception")
+
+    except VyperException:
+        pass
+
+    # rollback upon exception
+    assert m["x"] == 1