Summary:
The Xkb class loads keyboard layouts from the users configuration. This
makes tests fail locally if the user has a layout which behaves
differently to the one the test expects. E.g. on a German layout the
right alt key is different to the one of US layout.

In order to have a more stable test base the env variable
KWIN_XKB_DEFAULT_KEYMAP forces the loading of the default keymap, thus
tests have a common layout set.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2466