x/net/nettest: `TestableAddress` is not consistent with `TestableNetwork`

[bcmills]

What did you do?

Run the following program (https://go.dev/play/p/rwXVU67MVUC?v=gotip):

package main

import (
	"golang.org/x/net/nettest"
)

func main() {
	println(nettest.TestableNetwork("ip6"))
	println(nettest.TestableAddress("ip6", "::1"))
}

What did you expect to see?

Consistent answers: TestableAddress for an address on a given network should only be true if TestableNetwork is also true for that network.

What did you see instead?

false
true

Specifically, today TestableAddress always returns true for networks that are not one of unix, unixgram, or unixpacket.

(attn @neild @ianlancetaylor)