feat: add maybe_ versions of web socket receiving code

[JosephLenton]

Changes

• Adds TestWebSocket::maybe_receive_message()
• Adds TestWebSocket::maybe_receive_text()
• Adds TestWebSocket::maybe_receive_json()
• Adds TestWebSocket::maybe_receive_yaml()
• Adds TestWebSocket::maybe_receive_msgpack()
• Adds TestWebSocket::maybe_receive_bytes()
• Adds TestWebSocket::assert_receive_message()
• Adds TestWebSocket::assert_receive_no_message()
• Adds TestServerBuilder::web_socket_receive_timeout() -- to set the default timeout for waiting for web socket messages. Default is 20ms.

Breaking change

• All TestWebSocket receive functions will now timeout within 20 milliseconds if they didn't receive a message. Previously they would block forever until the stream is closed.

Implements #209

[JosephLenton]

What's left

1. Test flakiness when running test_assert_receive_text::it_should_ping_pong_large_text_blobs.
2. Missing test coverage for:

• assert_receive_no_message
• assert_receive_no_message_within
• assert_receive_message
• maybe_receive_text, maybe_receive_json, maybe_receive_yaml, maybe_receive_msgpack, maybe_receive_bytes
• set_receive_timeout and web_socket_receive_timeout builder methods

3. add documentation