The word "theory" implies that it is testable.
"Falsifiable" is a better word here. You don't need to be able to do controlled experiments (tests) in order to have a solid theory - an influx of new observations of the universe as we find it works just as well.
And the holographic principle is certainly falsifiable.
1) It imposes a limit on the amount of entropy in any given volume - find a system which can be in more than the allowed number of states, and isn't inside a black hole, and this theory is dead.
2) It sets a really high value on the entropy of black holes. Black holes become the dominant source of entropy in our universe. This has consequences in cosmology that are fundamental, if the only reason entropy is increasing in our universe is this assigning of entropy to black holes. There are certainly physicists playing with that idea, as it could be career-making, true or false.
3) It has deep implications for the evolution of black holes - how they evaporate. This will be a lot harder to prove (I don't think we'll validate Hawking radiation in my lifetime), but might be possible to falsify by finding a black hole that's clearly not allowed by theory.
Heck, there are implications for particle physics that are still being understood, and lots there is testable now with the LHC. The more and farther you reason from a premise like this, the more likely it is to matter to something easy to measure, or at least possible to measure.
The reason the discovery of the Higgs boson was such a big deal is that it confirmed a bunch of really abstract theory in quantum mechanics that is very, very far from anything we can measure, except at the end of this very long chain of reasoning there's this prediction of this new oddball particle (that there's no other reason to expect - it come from deep in the abstract math of QM, not from anything else we measured). So finding that particle confirms that whole crazy chain of logic. Something similar will eventually happen for the holographic principle.