You already have all the answers here,
Namely
You dont have any coolant hence no conditioners,
You do have an open pressure system (no cap) during test
You are literally seeing cavitation bubbles rise up.
Your coolant system is self bleeding so thats out
Headgasket breach allowing combustion pressure into cooling system doesnt show bubbles momentarily, it shows a constant foam (heavy foam) with cap off and excess air purging (with cap installed)
So from this list, what do you think you have?