tests: add test for signmessage and lightning-checkmessage.

Thanks Twitter helpers @duck1123 and @jochemin for tests!
And @bitconner for the initial test vector.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -1646,3 +1646,42 @@ def test_relative_config_dir(node_factory):
     assert os.path.isabs(l1.rpc.listconfigs()["lightning-dir"])
     l1.stop()
     os.chdir(initial_dir)
+
+
+def test_signmessage(node_factory):
+    l1 = node_factory.get_node()
+
+    corpus = [[None,
+               "this is a test!",
+               l1.rpc.call('signmessage', ["this is a test!"])['zbase'],
+               l1.info['id']]]
+
+    # Other contributions from LND users!
+    corpus += [
+        ['@bitconner',
+         "is this compatible?",
+         'rbgfioj114mh48d8egqx8o9qxqw4fmhe8jbeeabdioxnjk8z3t1ma1hu1fiswpakgucwwzwo6ofycffbsqusqdimugbh41n1g698hr9t',
+         '02b80cabdf82638aac86948e4c06e82064f547768dcef977677b9ea931ea75bab5'],
+        ['@duck1123',
+         'hi',
+         'rnrphcjswusbacjnmmmrynh9pqip7sy5cx695h6mfu64iac6qmcmsd8xnsyczwmpqp9shqkth3h4jmkgyqu5z47jfn1q7gpxtaqpx4xg',
+         '02de60d194e1ca5947b59fe8e2efd6aadeabfb67f2e89e13ae1a799c1e08e4a43b'],
+        ['@jochemin',
+         'hi',
+         'ry8bbsopmduhxy3dr5d9ekfeabdpimfx95kagdem7914wtca79jwamtbw4rxh69hg7n6x9ty8cqk33knbxaqftgxsfsaeprxkn1k48p3',
+         '022b8ece90ee891cbcdac0c1cc6af46b73c47212d8defbce80265ac81a6b794931'],
+    ]
+
+    for c in corpus:
+        print("Shout out to {}".format(c[0]))
+        assert subprocess.check_output(['devtools/lightning-checkmessage',
+                                        c[1], c[2]]).decode('utf-8') == "Signature claims to be from key {}\n".format(c[3])
+
+        subprocess.run(['devtools/lightning-checkmessage', c[1], c[2], c[3]], check=True)
+
+        with pytest.raises(subprocess.CalledProcessError):
+            subprocess.run(['devtools/lightning-checkmessage',
+                            c[1] + "modified", c[2], c[3]], check=True)
+
+        assert l1.rpc.call('checkmessage', [c[1], c[2], c[3]])['verified']
+        assert not l1.rpc.call('checkmessage', [c[1] + "modified", c[2], c[3]])['verified']