Standards for the development of safety-critical systems like ICE 61508, ISO 26262 (automotive), EN 50128 (railway), IEC 62304 (medical) require module respectively unit testing of the software of the (embedded) system. Many standards specify also the coverage measure that shall be determined depending on the safety integrity level of the product.
This webinar from Hitex explains unit testing ( What is a unit of a C program? What is the interface of a unit?) and the unit testing is demonstrated using the unit testing tool TESSY.