your metaphor is wrong.

code is not the output. functionality is the output, and you do look at that.

Explain then how testing the functionality (not the new one; regressions included, this is not a school exercise) is faster than checking the code.

Are you writing black box testing by hand, or manually checking, everything that would normally be a unit test? We have unit tests precisely because of how unworkable the “every test is black box” approach is.