A test mule is a prototype that is used for performance evaluation. It is a common term for preproduction cars, but is also widely used in non-automotive product development. Where did the term come from? I have asked product developers in several industries and they don't know. There are also pony engines and steam donkeys, and engines generally replaced horses for power production, but I can't find the origin of test mules. One possible origin is that a mule was used as direct comparison for evaluating the power of something such as a wind mill or water wheel. Records of tests of machinery vs animals go back at least a thousand years, but the test mule appears to be recent enough to hope for a first-person account.
It may have originated in Italian rather than English, because the 1964 Road and Track vol. 16, page 43 says:
had built a pair of muletti — "mules" — whose design had been hastily roughed out by the same internal talent that had drawn up the Dischi Volanti and many other "house" designs. The workmanship of these muletti also was rough as they were never intended to be seen by the public.
There are slightly earlier examples in English:
After this crash, the practice/testing "mule" was hauled out, the salvageable pieces from Gurney's previous mount installed. On the day following the crash, Dashing Dan qualified at 149.019-mph, what was ultimately to be 17th fastest.
With a rough fiberglass body this became the "Mule", which went down to Sebring for on-the-spot trials while the actual race car was completed... the Mule was revised and cleaned up in detail to be exactly like the race SS, but the ax fell on the project before the ex-Mule could be assembled ... this car, the Mule...
...while the SS in both "Mule" and "show" variants ran they went like stink. The officially released lap time set by Fangio at Sebring in the prototype Mule was 3:27.2, a very respectable figure.
Chevrolet's practice car had fiberglass body, was called "the mule"
There is earlier literal use of "test mule", such as in the 1955 Journal of the American Veterinary Medical Association, Volume 126, page 285
When test mule 1434 and test horse 1121 were subsequently challenged with a known strain of virus, both promptly developed clinical symptoms of infectious anemia
In French or Italian, car racing teams were/are called "écurie (de course)" or "scuderia", literally racing stable: the race cars are the horses and the replacement car is called in French "le mulet" (the mule) and in Italian "il muletto".
Both in French and in English, the sense of mule/mulet later extended to development cars (testbed vehicle equipped with prototype components requiring evaluation).
The italian Wikipedia article on muletto refers to a French origin of that Italian word, because in italian, the animal is a "mulo".
However, I have any other evidence to support a French/Italian origin of "mule" in the sense of evaluation prototype. Even if the word is quite common in French, I only found one occurence in the very view Google books on car racing: the book was written in 1958.