What is the expiation for my lying to avoid undergoing a certain test, despite my intention not to benefit from the tests for which health insurance paid?

Lying is forbidden and is one of the characteristics of hypocrites. It leads to immorality and Hellfire. Therefore, a Muslim must avoid it and repent to Allah from it by feeling remorse, abandoning it, and resolving never to return to it. Repentance from lying does not require expiation; rather, it is like repenting from all other forbidden acts between a servant and his Lord. What you mentioned regarding insurance has no sin in it.