[autotests/integration] Use socketpair for Wayland connection
Summary:
This is a preparation step for no longer creating a socket in the tests
and slightly simplifies the init test code.
Reviewers: KWin, Plasma on Wayland
Subscribers: plasma-devel, kwin
Tags: Plasma on Wayland, KWin
Differential Revision: https://phabricator.kde.org/D3575