TCP smoke testing

[yoshuawuyts]

Currently our system isn't running automated tests for TCP. We should at least have some smoke tests for:

• async_std::net::TcpListener + std::net::TcpStream (test tcp server)
• async_std::net::TcpStream + std::net::TcpListener (test tcp client)

There's an empty tests/tcp.rs file which we could use to fill in. This should be quite straight forward when basing it on the examples. The only tricky part would be how to do shutdown, but dropping a task should work well enough here (: Marking this as a good first issue!

[markhildreth]

It seems that tests/tcp.rs is not empty, but does already does have some tests in it, although none of them appear to be using the std library's TcpStream/TcpListener. To be more specific, are you looking for a test or two that has an async_std stream connect to a std listener, and a std stream connect to an async_std listener?

[markhildreth]

I've added #429 based on my assumption of the requirements. Please let me know if I've misunderstood the requirements. If so, I can make revisions.