If you code in OOP with the concept of SoC and SOLID you have the possibility to write modern modular classes.

Take the focus on the testability of the code you write and it will show you the way which you can/must go.

1. #unittest
2. #functionaltests
3. #integrationtests
4. #apitests
5. #acceptancetests

From top to bottom
From many to few tests