efi_selftest: test task priority levels

Run a 10 ms periodic timer and check that it is called 10 times
while waiting for 100 ms single shot timer.

Raise the TPL level to the level of the 10 ms timer and observe
that the notification function is not called again.

Lower the TPL level and check that the queued notification
function is called.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>