[autotests] Add tests for DRM platform plugin

Summary:
The addition of the test infrastructure is motivated by the regressions
caused by adding mode switching and transformation support.
A contributing factor to these regression is the fact that the DRM
platform does not have any tests. It is difficult to test this code as
it needs to work with hardware, thus we cannot use the real DRM library.
Instead we need to use mocking.

This change sets up some first basic tests with the help of a mockDrm
library. In order to better test the code as units the Drm classes are
slightly refactored. Most importantly the dependency to DrmBackend is
removed wherever possible and replaced by a simple int fd which is mostly
the only element used by the classes.

This first test introduces basic testing of a DrmObject. It is intended
to extend this to at least also test DrmPlane as a central piece of our
Drm platform plugin. This will also extend the tests of DrmObject.

Reviewers: KWin, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8776