is there any testable proof that single celled organisms became multi cellular?
While we cannot go back in time to actually view the transition of single-celled organisms into multicellular ones, we can see evidence of such a change in the fossil record and in living organisms existing today that represent those transitional forms. One commonly cited example of the tendency of living cells to aggregate can be seen in the volvocine line of algae:
Note that no one is saying that Chlamydomonas (the single-celled algae) evolved into Gregarina (a loose aggregate of Chlamydomonas-like cells) or that either Chlamydomonas or Gonium evolved into Pandorina (a more organized colony of Chlamydomonas-like cells) or that any of those evolved into Volvox (an incredibly organized little "Death Star" of cooperative Chlamydomonas-like cells with a division of labor). It can be said only that all these species share a common ancestor, and that the descendants of that ancestor acquired new, adaptive traits with respect to cell aggregation and branched into the various living descendants we see today.
Probably a major step in the evolution of multicellularity was the appearance of protein cadherins and catenins, which play important roles in cell adhesion:
These proteins evolved from protein precursors found in single-celled organisms.
It is evident that multicellularity arose several times in different eukaryotic lineages:
You can find one of many overviews here:
You also can find some very well researched articles about the evolution vs. creation arguments for multicellularity here:
Hope that helps!