Formula 1 has this rule of banning off-season (Apart from the official F1 approved tests at the end of February or the start of March) and in-season testing. This is done mainly to reduce costs to Formula 1 team owners and make their existence in the sport sustainable.

Source: This

Of all the different rules changes which have been enacted in Formula 1 in recent years, perhaps none has been as controversial as the ban on in-season testing that started in 2009. The move was done to help to control costs as the sport dealt with the reality of economics in a global recession.

And this.

Most, but maybe not all, teams will have their new cars ready to hit the track as soon as the off-season testing ban is lifted - by which time they should also have a clearer picture of what the future holds.

There are a few teams that own private racetracks (For example: Ferrari owns the Fiorano Circuit). How do Formula 1 administrators ensure that testing does not take place on these circuits at any times apart from the ones on the official itinerary?