Add note about JSON tests maybe being wrong. (#1992)

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.
diff --git a/.gitignore b/.gitignore
index 8f12f22..052dd14 100644
--- a/.gitignore
+++ b/.gitignore
@@ -111,7 +111,10 @@
 conformance/Conformance.pbobjc.h
 conformance/Conformance.pbobjc.m
 conformance/conformance_pb.rb
+conformance/failing_tests.txt
 conformance/google/
 conformance/javac_middleman
 conformance/lite/
+conformance/nonexistent_tests.txt
 conformance/protoc_middleman
+conformance/succeeding_tests.txt