Use test setup methods (methods that are annotated with @testSetup) to create test records once and then access them in every test method in the test class. Test setup methods can be time-saving when you need to create reference or prerequisite data for all test methods, or a common set of records that all test methods operate on. Test setup methods can reduce test execution times especially when you’re working with many records. Tes t setup methods enable you to create common test data easily and efficiently. By setting up records once for the class, you don’t need to re-create records for each test method. Also, because the rollback of records that are created during test setup happens at the end of the execution of the entire class, the number of records that are rolled back is reduced. As a result, system resources are used more efficiently compared to creating those records and having them rolled back for each test method @isTest private class CommonTestSetup { @...
“ It's hard enough to find an error in your code when you're looking for it; it's even harder when you've assumed your code is error-free. ”