integration: test /ts2021 WebSocket GET with a real WASM client

A raw coder/websocket dial and the real tailscale.com js/wasm control client under Node, both against headscale alongside normal Tailscale clients.

Updates #3357

integration/wasmic/wasmclient/stub.go

@@ -0,0 +1,8 @@
+//go:build !js
+
+// This package only does something when built for GOOS=js (see main.go). The
+// stub exists so `go build ./...` and `go vet ./...` on the host don't fail with
+// "build constraints exclude all Go files" for this directory.
+package main
+
+func main() {}