I take "prueba" here to mean a race or event rather than a test. Although the term "regularity test" is certainly found sometimes, this type of event is much more commonly called a "regularity rally" in English.

"Regularity rally
Regularity rallying, also known as TSD rallying (Time Speed Distance), is a form of motor sport usually conducted on public roads but sometimes including off-road and track sections. The object of these rallies is to maintain precise times and precise average speeds on various segments of a predefined route. Each team usually consists of an amateur driver and navigator. Teams usually start a regularity rally at fixed intervals creating a field that is spread along the route. [...]
A typical regularity rally may run for a few hours or it may run over a series of stages over a few days."
http://en.wikipedia.org/wiki/Regularity_rally

The variant "regularity test", though used in some bona fide English sources, may be a calque, possibly from German:

"A Gleichmäßigkeitsprüfung (abbrev. GLP, German for regularity test) is a motorsport event where precise timing is winning, not outright speed.
Mainly Classic cars events like the modern Mille Miglia are held as GLPs."
http://en.wikipedia.org/wiki/Gleichmäßigkeitsprüfung

--------------------------------------------------
Note added at 8 hrs (2011-05-18 13:40:02 GMT)
--------------------------------------------------

Further research confirms that "regularity rally" is by far the most common term overall, and overwhelmingly so for classic cars, where "historic regularity rally" is virtually the only term in use. It is also the term favoured by the FIA (Fédération Internationale de l'Automobile):

http://www.fia.com/en-GB/sport/championships/Pages/rally_reg...