net/http: add test coverage for Fetch API-based Transport when viable

[dmitshur]

In #57613 (comment), I wrote:

[...] the main reason the fake socket path was used is because that was the easiest path forward at the time. Back then, the NaCL port had the fake socket code implemented, and Node didn't support fetch. By now, with NaCL port being long gone and Node supporting fetch, it might be net simpler to start using fetch. But that is fine to investigate and consider doing if desired later.

Just disabling fetch during tests with Node 18 as done in CL 463976 is good to resolve the immediate issue. Thanks.

Now that the immediate issue is resolved, this is the tracking issue for the "investigate and consider doing" part.

In addition to possibly being simpler, I think the main benefit here would be if it lets us have coverage for the Fetch API-based Transport on the builders, and help catch regressions like:

• net/http: Fetch API-based Transport is accidentally disabled too widely #60808
• net/http: Fetch API-based Transport panics when request has no body #60809

CC @golang/js, @golang/wasm.