Factored Conformance and Benchmark test messages into shared test schema. (#1971)

* Factored Conformance test messages into shared test schema.

* Updated benchmarks to use new proto3 message locations.

* Fixed include path.

* Conformance: fixed include of Python test messages.

* Make maven in Rakefile use --batch-mode.

* Revert changes to benchmarks.

On second thought I think a separate schema for
CPU benchmarking makes sense.

* Try regenerating C# protos for new test protos.

* Removed benchmark messages from test proto.

* Added Jon Skeet's fixes for C#.

* Removed duplicate/old test messages C# file.

* C# fixes for test schema move.

* Fixed C# to use the correct TestAllTypes message.

* Fixes for Objective C test schema move.

* Added missing EXTRA_DIST file.

Makefile.am

diff --git a/Makefile.am b/Makefile.am
index 0d8fe10..2734378 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -100,6 +100,7 @@
   csharp/src/Google.Protobuf.Test/TestCornerCases.cs                         \
   csharp/src/Google.Protobuf.Test/TestProtos/ForeignMessagePartial.cs        \
   csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs            \
+  csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs           \
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs         \
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs   \
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs               \