I think any question of the form "Assume X is ethical, is X' also ethical?" is inherently malformed. If my ethics do not follow X, then the change in my ethics which causes me to include X may be very relevant to X'.

I don't think anyone who is a vegetarian regardless of self-awareness would be able to answer the question you are asking.

I think the big question that implies this one is "Should we eat baby humans? Why?"

I believe the answer is "No, because there is no convenient place to draw the line between baby and adult, so we should put the line at the beginning, and because other people may have strong emotional attachment to the baby."

I think the first part of my reason is eliminated by your "reliable test." If the test is completely reliable, that is a very good place to draw the line.

The second part is not going away. It has been evolved in us for a very long time, however, it is not clear if people will get the same attachment to non-human babies. I think that our attachment to non-humans is much lower, and there is not a significant difference between their attachment before and after self awareness.

However, the question asked assumes that our ethics distinguish between creatures with and without self awareness. If that distinction is caused by us having different levels of emotional attachment to the animal depending on its self awareness, then it would change my answer.