All life support systems built to date have either relied on a set amount of consumables available at launch (no resupply, such as on Mercury, Gemini, Apollo, Skylab, the Shuttle, Soyuz) or on being able to resupply to make up for losses (some Salyut, Mir, and the ISS). Resupply isn't viable on a Mars mission except for the hope of using materials from the Martian atmosphere. However, you have to look at what happens should the mission have to abort and return to Earth without landing on Mars. In that case, the supplies you have on board at the beginning of the mission have to last upwards of 2 years in the worst case scenario. However, none of our existing technologies can last that long. For example, there are issues with the long-term storage of cyrogenic liquids like LOX. These issues must be solved before we can send people to Mars.

There are several ideas out there for building such a life support system. Some call for recycling all wastes, growing plants, etc., to provide a significant percentage of the consumables. These ideas sound good, but they need to be developed and tested in space. Again, an engineer has to look into failure modes and how to keep the crew alive should something like crop failure happen.

Some of these ideas could be tested on the ISS, but a real test would require a dedicated module. The current ISS life support system still requires resupply, and what good is the test if you're getting stuff from the ground. It's be as bogus as the Biosphere II test where they opened windows to let out excess CO2 and let in more O2 when the CO2 levels got too high.

Personally, I'd like to test the systems by putting them into a multi-function vehicle and flying missions of increasing length and complexity. This might start in LEO, then expand out to the moon for stays of increasing duration, then perhaps going out to visit a NEO. That's how flight test is done - incremental expansion of the envelop. The days of test pilots strapping on a plane and diving until they rip the wings off are the stuff of ancient Hollywood fantasy (see "Test Pilot" starring Clark Gable). Incremental expansion of the envelop is how testing is done in the real world. Boeing and Airbus don't put paying passengers on their new airliners' first flights for a reason.