I have always wished for my computer to be as easy to use as my telephone; my wish has come true because I can no longer figure out how to use my tel… - Bjarne Stroustrup

"How to test?" is a question that cannot be answered in general. "When to test?" however, does have a general answer: as early and as often as possible.

[Corporate programming] is often done to the point where the individual is completely submerged in corporate "culture" with no outlet for unique talents and skills. Corporate practices can be directly hostile to individuals with exceptional skills and initiative in technical matters. I consider such management of technical people cruel and wasteful.

The connection between the language in which we think/program and the problems and solutions we can imagine is very close. For this reason restricting language features with the intent of eliminating programmer errors is at best dangerous.