Write as much code as you can that has no dependencies. Unit test that code exhaustively. Fake all inputs that don't contain behavior. Mock all interactions that do. Then write functional tests that check that the glue and state management actually work with the real things.
The plumber is still going to run water and check for leaks before they leave, no matter how many certifications the copper piping came with. But that's only at the end of a long process of work and inspections.
Nothing pisses me off like finding a suite of tests that has fakes with logic in them. By the time I find them, the fakes are longer than the tests. Often the commit history shows that this accumulated by accretion, and nobody ever pulled the emergency stop lever. Other times it's people who are wrong-headed about what problems tests are trying to solve (coverage chasers are but one category).
Write as much code as you can that has no dependencies. Unit test that code exhaustively. Fake all inputs that don't contain behavior. Mock all interactions that do. Then write functional tests that check that the glue and state management actually work with the real things.
The plumber is still going to run water and check for leaks before they leave, no matter how many certifications the copper piping came with. But that's only at the end of a long process of work and inspections.