This commit adds the js/wasm architecture
to the internal/nettest package.

js/wasm currently uses the same fake networking as nacl.
Support for real networking will be added later.

Related to https://go-review.googlesource.com/c/go/+/109995

Change-Id: Ic914175374d7588dcb3c078da0e0e71c88323d1a
Reviewed-on: https://go-review.googlesource.com/110895Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>