Add tests to test_exports that check allcaps constants re-exports

[njsmith]

#791 added a test in test_subprocess.py to check that trio.subprocess is re-exporting all the constants from subprocess. We should move this to test_exports.py and generalize it to check trio.ssl and trio.socket, as well. (Or maybe we already have some tests in there for that? If so we should combine them instead of having multiple ad hoc duplicates of the same thing.)

[stevenjackson121]

@njsmith said in chat

we deprecated trio.subprocess and trio.ssl. But, we didn't deprecate trio.socket, so there might still be something that needs doing?

I'm assuming (if this issue is still relevant) we want to just add the tests for trio.socket in test_socket.py rather than moving it out to test_exports.py for the singular remaining case. If a different approach is preferred, please let me know!