#[multilib-testing]
#Include = /etc/pacman.d/mirrorlist

#[multilib]
#Include = /etc/pacman.d/mirrorlist

sed -i 's/#[multilib]/[multilib]/g' /etc/pacman.conf

sed -i 's/#Include =/Include =/g' pacman.conf

sed '/#[multilib]/{N;s/n#/n/}' file

#[multilib-testing]
#Include = /etc/pacman.d/mirrorlist

#[multilib]
Include = /etc/pacman.d/mirrorlist