Either you KNOW you didn't introduce any new bugs or you don't know and are relying on hope. Testing makes the difference. I really do not care how you arrive

Either you KNOW you didn't introduce any new bugs or you don't know and are relying on hope. Testing makes the difference. I really do not care how you arrive to 100% certainty, just get there. If it means testing some structural part of your code (and other unit-testing), or it means settings up integration tests that do end to end tests on live systems, just get there. We can talk about increasing efficiency in this process as a team. For some systems it might be easier to do integration tests, for others its more efficient to do unit-tests.