http2: don't rely on double-close of a net.Conn failing
http2: don't rely on double-close of a net.Conn failing

The js_wasm net.Conn implementation doesn't return an error on
double-closing a connection. Update tests to not rely on this
behavior.

Change-Id: I674c6cd6364b7351d627626cf5bd9f59b3c2b96c
Reviewed-on: https://go-review.googlesource.com/c/net/+/432515
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/net