x/net/nettest: persistent failures with `tcp is not supported` on dragonfly and js/wasm since CL 458096

[bcmills]

#!watchflakes
post <- pkg ~ `^golang.org/x/net` && `tcp is not supported on `

Examples:
https://build.golang.org/log/0b1e67c60e7fc377a0c0a97bf1d4bd2fc3b98770
https://build.golang.org/log/890bc6ff0dc90108fe55e5b247beeda5863c119c

[gopherbot]

Change https://go.dev/cl/460715 mentions this issue: Revert "nettest: use RoutedInterface for probing network stack capability"

[bcmills]

I've mailed a revert in https://go.dev/cl/460715.

[gopherbot]

Change https://go.dev/cl/460735 mentions this issue: nettest: fix tests on dragonfly and js/wasm