[C#] Update test project to be used with dotnet test and Visual Studio Test Explorer

* Fix setup.py for windows build.

* Bump version number to 3.5.2

* Cat the test-suite.log on errors for presubits

Improve error message when googletest is missing.

Add support for Power (ppc64le) arch

Cleaner protoc artifacts

Update instructions about getting protobuf source.

Move ObjC WKTs to minimal imports to avoid import cycle.

Since the generated header import GPBProtocolBuffers.h, there is no need
to generate imports for the WKTs as they will have already been imported.

To ensure all headers aren't dependent on other things being imported
before/after them, make a source that just imports each header and add
it to the unittesting target, that way we ensure it can be included on
its own with ordering issues.

Also do this testing with a few generated headers that aren't part of
the library to help ensure the different generated imports needed are
complete.

To avoid a cycle between headers, have the WKTs use minimal imports instead
of using the helper to get everything from the library.

Fixes https://github.com/google/protobuf/issues/4301
Fixes https://github.com/google/protobuf/issues/4403

ENTRYPOINT is used even when other commands are specified on the "docker
run" command line. This allows running one-off commands in the docker
image (especially combined with volume binding with the host) with the
correct environment variables.

This is just better Dockerfile hygene as it produces smaller resulting
images with fewer useless files.

It is a bad idea to check out code into the docker image, as it will be
out-of-date. It is better to have the image just be the environment, and
let any scripts that need source check them out themselves.

This fixes #4419 in that it allows the image to build again, albeit
users would need to use wget to grab the source of the version of
protobuf they wish.

Cleanup LICENSE file.

Add kokoro build status badges.

Moved the badges to the language table as they are per-language based.
Some badges are not yet available due to stuck kokoro jobs.

Delete unused directories.

jenkins has been replaced by kokoro and we never use more_tests.

Removed atomicops.h stub

Change C# reflection to avoid using expression trees

For oneofs, to get the case, we need to call the property that
returns the enum value. We really want it as an int, and modern
runtimes allow us to create a delegate which returns an int from the
method. (I suspect that the MS runtime has always allowed that.)
Old versions of Mono (e.g. used by Unity3d) don't allow that, so we
have to convert the enum value to an int via boxing. It's ugly, but
it should work.

This should work on Unity, Mono and .NET 3.5 as far as I'm aware.
It won't work on platforms where reflection itself is prohibited,
but that's a non-starter basically.

Delete javanano kokoro build configs.

Remove broken build status icons.

Remove copyright attributions to files that no longer exist; Change the
year to 2008 to be consistent with the copyright header of other source
files.

Their job configs are being deleted in cl/190526138

Add googletest as a submodule in third_party/googletest.

* when interpreting options, rewrite file descriptor's source code info
- so that interpreted option paths have correct location information
- so that corresponding uninterpreted option paths are removed
also includes a fix to source code locations for extension range options

Remove javanano.