make nhooyr.io/websocket/internal/test/wstest public

[zhangyoufu]

This package is useful for anyone writing WebSocket-related test cases. But it is an internal package, thus cannot be used outside nhooyr.io/websocket.

I think this package should be moved out of internal, like the relationship between net/http and net/http/httptest.

[zhangyoufu]

duplicate of #197

[nhooyr]

Yea, definitely a very useful package, happy there's demand for it :) will def close out #197 sooner.