non-obvious choices in std IPv6 test creation

[workingjubilee]

re: https://github.com/rust-lang/rust/blob/9fb94b32df38073bf63d009df77ed10cb1c989d0/library/std/src/net/udp/tests.rs

We make a bunch of devices with a scope_id of 0, but the value of 0 is a reserved scope_id. I believe it's usually ignored or treated as equivalent to "eh whatever", but we probably should be making them with scope_id: 1, i.e. "entirely local to here". This prompted #137967 because AIX helpfully fixes the value of scope_id to the one it interprets it as: 1.

[bjorn3]

When parsing a string like ::1 we also use scope id 0 when there is no explicit scope id specified:

rust/library/core/src/net/parser.rs

Line 279 in 374ce1f

let scope_id = p.read_scope_id().unwrap_or(0);