Implement support for registry integration tests via py.test

This change implements support for registry integration tests using the new py.test-based live server test fixture. We can now parametrize the protocols we use (in prep for V2_2), and it makes the code *much* cleaner and less hacky.

Note that moving the vast majority of the tests over from the existing impl will come as a followup PR

test/registry/protocol_fixtures.py

@@ -0,0 +1,35 @@
+import pytest
+
+from Crypto.PublicKey import RSA
+from jwkest.jwk import RSAKey
+
+from test.registry.protocols import Image, layer_bytes_for_contents
+from test.registry.protocol_v1 import V1Protocol
+from test.registry.protocol_v2 import V2Protocol
+
+
+@pytest.fixture(scope="session")
+def basic_images():
+  """ Returns basic images for push and pull testing. """
+  # Note: order is from base layer down to leaf.
+  return [
+    Image(id='parentid', bytes=layer_bytes_for_contents('parent contents'),
+          parent_id=None, size=None),
+    Image(id='someid', bytes=layer_bytes_for_contents('some contents'),
+          parent_id='parentid', size=None),
+  ]
+
+
+@pytest.fixture(scope="session")
+def jwk():
+  return RSAKey(key=RSA.generate(2048))
+
+
+@pytest.fixture(params=[V1Protocol, V2Protocol])
+def pusher(request, jwk):
+  return request.param(jwk)
+
+
+@pytest.fixture(params=[V1Protocol, V2Protocol])
+def puller(request, jwk):
+  return request.param(jwk)