selftests: net: unify the Makefile formats

selftests: net: unify the Makefile formats

We get a significant number of conflicts between net and net-next
because of selftests Makefile changes. People tend to append new
test cases at the end of the Makefile when there's no clear sort
order. Sort all networking selftests Makefiles, use the following
format:

 VAR_NAME := \
	 entry1 \
	 entry2 \
	 entry3 \
 # end of VAR_NAME

Some Makefiles are already pretty close to this.

Acked-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Acked-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20251003210127.1021918-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>