How to test legacy software (that was not designed to be tested) when modernising

You might have encountered software that is not easily testable, and by this I mean software that is not tested from the outset. You might have even tried to test it, but failed to do so. Don’t worry, software where testing is an after-thought is never going to be as testable as software designed to be...

How to test legacy software (that was not designed to be tested) when modernising