pytest: Add tests for the sphinx onion generation and processing

These just run the test vectors and add a test for the devtools/onion tool so
we don't accidentally break them.

tests/test_onion.py

@@ -0,0 +1,60 @@
+import subprocess
+import pytest
+import os
+from fixtures import *  # noqa: F401,F403
+
+
+@pytest.fixture
+def oniontool():
+    path = os.path.join(os.path.dirname(__file__), "..", "devtools", "onion")
+    return path
+
+
+privkeys = [
+    '4141414141414141414141414141414141414141414141414141414141414141',
+    '4242424242424242424242424242424242424242424242424242424242424242',
+    '4343434343434343434343434343434343434343434343434343434343434343',
+    '4444444444444444444444444444444444444444444444444444444444444444',
+    '4545454545454545454545454545454545454545454545454545454545454545'
+]
+
+pubkeys = [
+    '02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619',
+    '0324653eac434488002cc06bbfb7f10fe18991e35f9fe4302dbea6d2353dc0ab1c',
+    '027f31ebc5462c1fdce1b737ecff52d37d75dea43ce11c74d25aa297165faa2007',
+    '032c0b7cf95324a07d05398b240174dc0c2be444d96b159aa6c7f7b1e668680991',
+    '02edabbd16b41c8371b92ef2f04c1185b4f03b6dcd52ba9b78d9d7c89c8f221145'
+]
+
+
+def test_onion(directory, oniontool):
+    """ Generate a 5 hop onion and then decode it.
+    """
+    os.makedirs(directory)
+    tempfile = os.path.join(directory, 'onion')
+    out = subprocess.check_output(
+        [oniontool, 'generate'] + pubkeys
+    ).decode('ASCII').strip().split('\n')
+    assert(len(out) == 1)
+
+    def store_onion(o):
+        with open(tempfile, 'w') as f:
+            f.write(o)
+
+    store_onion(out[0])
+
+    for i, pk in enumerate(privkeys):
+        out = subprocess.check_output([oniontool, 'decode', tempfile, pk]).decode('ASCII').strip().split('\n')
+        store_onion(out[-1][5:])
+
+    assert(out == ['payload=000000000000000000000000000000000400000004000000000000000000000000'])
+
+
+def test_onion_vectors(oniontool):
+    tests = [
+        'onion-test-multi-frame.json',
+        'onion-test-v0.json']
+
+    for t in tests:
+        p = os.path.join(os.path.dirname(__file__), 'vectors', t)
+        print(subprocess.check_output([oniontool, 'runtest', p]).decode('ASCII'))