[autotest] Introduce a test infrastructure for GLPlatform

The new test can load "profiles" from kconfig files in the test data.
Based on that the glGetString return values are mocked and GLPlatform
can perform detect without having to interact with a real GL library.

That way we can verify that the detect code works correctly. As a first
test the settings of one Intel/IvyBridge is included. More tests can be
added easily (e.g. looking at various supportInformation output in
bugs.kde.org). Also this allows to more easily add detect code for GPUs
we do not know yet. And to simulate conditions where the detect code
failed resulting in no compositing at all.