For the past quarter century, Intuit, the maker of TurboTax, has worked to thwart one clear threat to its profits: a free, publicly funded tool to file taxes online. The company’s success at preventing that threat was near total — until earlier this year, when the IRS announced a plan to test such an approach. Advocates cheered, seeing it as a first step to a system where Americans, particularly low-income taxpayers, could easily avoid paying big fees for tax preparation.