tests(s2n-tls-hyper): Gracefully shutdown client in tests

[goatgoose]

Problem:

The hyper client isn't currently capable of shutting down gracefully, and immediately closes its socket after sendings its shutdown to the server. This causes the server to emit a NotConnected socket error when attempting to send its shutdown to the client.

#4838 adds a s2n-tls hyper server for localhost testing purposes. Since the s2n-tls hyper client can't be configured to shutdown gracefully, the server must be configured to ignore NotConnected errors.

Solution:

After the hyper client can be configured to shutdown gracefully, this should be implemented in the s2n-tls-hyper tests. Then, the NotConnected exception should be removed from the test server.

[goatgoose]

Issue opened to hyper: hyperium/hyper#3775