selftests: net: add IPv4 and IPv6 address order check

Introduce the following new tests: `ipv4_verify_same_scope_addr_order`,
`ipv4_verify_inter_scope_addr_order`, `ipv6_verify_same_scope_addr_order`,
`ipv6_verify_inter_scope_addr_order` to check the ordering of a
set of IP addresses after being inserted.

The implementations of these protocols are inconsistent
in regard to address ordering. IPv4 addresses stay in the
same order as inserted while IPv6 addresses appear in
reverse order. This inconsistency has prompted attempts
to unify the ordering, so both protocols act the same
(as IPv4). This however caused user-space regressions in
certain applications, which relied on the order as it
was prior to the change (particularly NetworkManager).
A similar inconsistency occurs when inserting different
scope addresses, where IPv4 puts link local ones before global
and IPv6 does the opposite. Attempts were already made
to unify this also, risking further regressions.

The addition of these tests aims to consolidate current
behaviour to prevent regressions in the future.
The expected behaviour is the initial one, where each
protocol acts differently. The same goes for inter scope
addresses.

Conversations detailing the decision processes for creating
these tests are linked below.

Link: https://lore.kernel.org/netdev/20260521135310.GC977@cmadams.net/
Link: https://lore.kernel.org/netdev/20260529112357.5079-1-fmancera@suse.de/
Link: https://lore.kernel.org/netdev/20260721090114.GA2510713@shredder/
Suggested-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Martin Jabůrek <martin.jaburek@suse.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260914065848.10518-1-martin.jaburek@suse.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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