Testut was born in Lyon, France, and later settled in Monaco, where he became a citizen of the principality. His connection to the local motor racing community was shaped in part by Louis Chiron, the celebrated Monegasque driver already well established in the sport by the time Testut began racing. Testut appears to have attempted an entry at the 1948 Bol d'Or at the age of 22, though records suggest either the car did not arrive or he did not ultimately start.
Testut's confirmed racing debut came in September 1956 at the Course de Côte de Vuillafans-Echevanne in France, where he drove an O.S.C.A. MT4 roadster and finished second. Chiron competed at the same event. Later that month Testut added a third-place finish at the Coupe d'Automne driving a Porsche 356.
In 1957 he partnered Chiron for the 24th Mille Miglia, the pair racing a Citroën DS19 under the Montecarlo Team name and finishing 103rd. That same year Testut won the Vuillafans-Echevannes sports car event outright, breaking Chiron's existing record on the course. The Maserati brothers, who had by this point founded OSCA after their departure from the company bearing their name, sent Testut a congratulatory letter in recognition of the achievement.
Encouraged by his sports car success, Testut acquired Maserati 250F chassis 2521 — a car that had raced for the Maserati factory in 1956 and subsequently run under the Ecurie du Puy team before being sold. Testut returned the car to the Maserati factory for updated bodywork before preparing for his Formula One campaign.
He entered the Formula One World Championship at the Monaco Grand Prix in both 1958 and 1959. On each occasion he drove the 250F and failed to qualify. In 1958 he also entered a second car at Monaco for Chiron; both drivers failed to qualify, marking the end of their Monaco racing careers.
Outside the World Championship, Testut showed more genuine pace. At the 1958 Syracuse Grand Prix he ran as high as seventh with ten laps remaining before retiring with engine failure — a performance that suggested he was competitive against the broader privateers field, even if unable to match the pace required for Monaco qualification. His result at Syracuse placed him in contention at a race that, while non-championship, attracted a reasonably representative entry. His practice times at Monaco in 1958 placed him 23rd in a large field of qualifying hopefuls.
Testut died in September 2005. He remains one of the minor figures of the late 1950s Formula One privateer scene, notable chiefly as one of only a handful of Monegasque-resident drivers to have attempted the Monaco Grand Prix during the sport's earliest championship years.