Add note about JSON tests maybe being wrong.

- Add note about the JSON test maybe not being correct yet.
- Add test to checks the generated names for double underscores to be sure they
  are what is expected.

- String behaviors
- Testing only valid JSON is allowed

The failure lists were falling out of date because the
tests would pass even if people forgot to remove failures
from the list.

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

Change-Id: I382dcda97fa123a6da4ff5faad5d7ece95853f33

This allows us to enable conformance tests even when we know
that some tests are failing and need to be fixed.

Change-Id: I372f43663008747db6f2b2cf06e6ffa4c6d85b2d