Hey Damyan,
I agree with your proposal and with all the other comments in this thread.
One thing I do want to note:
* export AUTOMATED_TESTING=1 when invoking the test system
It's worth mentioning that we have been doing this for a long time as pkg-perl policy anyway, so for our packages there's no real change (except that we can stop overriding dh_auto_test to also add the AUTOMATED_TESTING environment variable).
I think that it's a sensible change, especially given the reference you give re: the Lancaster consensus.
Overriding the variable should be relatively easy, right? Just unset that env variable or force it to some value?