This was enabled by the recent open-sourcing of JSON
support and MessageDifferencer.

MessageDifferencer allows the conformance suite to expand
because it allows us to write tests for payloads that parse
successfully.  To verify the testee's output payload, we
need to parse it back into a message and compare the message
instances.  Comparing output bytes vs. a golden message is
*not* valid, because protobufs do not have a canonical
encoding (especially in the presence of maps, which have
no prescribed serialization order).

We only add one small JSON test for now, but with the
framework in place we now have the foundation to dramatically
expand the coverage of the conformance test suite.

Also added the ability for the testee to skip tests that
exercise features that are unimplemented.  This allows
Java (which currently has no JSON support) to skip tests
involving JSON.

Change-Id: I697b4363da432b61ae3b638b4287c4cda1af4deb