Make XdgTest spec compliant
Summary:
Right now it worked in kwin, but was in fact breaking the spec sending
buffers before it was configured.
There also seems to be an unclearly written (but very very sensible)
rule about comitting the surface after initial properties in order to
recieve the configure event.
After these changes it works in Weston too.
Test Plan: Ran test in kwin_wayland and weston
Reviewers: KWin
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D18577