C#: Rename integration test directories.

We are no longer bound to the platform-specific directories, so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.

This is not very invasive for C#, I'm just dropping the `only`
suffix. You could also merge all the platform-specific test dirs,
or all test dirs into the top-level directory. I'll leave that up to you.

Author: criemen