Replaces the existing local loopback check with a check to see
whether the program is being interpreted by Node. This means
tests that are run with Node will use the fake network while still
allowing users who are using js/wasm to talk to local networks.

Updates #25506

Change-Id: I8bc3c6808fa29293b7ac5f77b186140c4ed90b51
GitHub-Last-Rev: 43d26af7bc716b7a01dd8f47d7a2c2a2df549489
GitHub-Pull-Request: golang/go#25663
Reviewed-on: https://go-review.googlesource.com/115495Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>