Its testable just not with current tech
What technology can you imagine that could ever prove or disprove a hypothesis of our universe being only one facet of a multiverse? The apparatus would necessarily exist within the closed system.
does being in a multiverse influences our world in any way using current theories?
yes > we can eventually figure out and run an experiment no > then studying it is as productive as searching the Russel's teapot
this is how you set apart a theory from speculation, a falsification test. i.e. > http://arxiv.org/abs/1210.1847