drm/tests: managed: Add comments about test intent

Add comments explaining the intention behind the test and certain
implementation details related to device lifetime.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240115171351.504264-4-michal.winiarski@intel.com