You still have to test all the 80 lines if they're broken down into multiple functions, so it's something that you have to evaluate on a case-by-case basis.
It might even make it harder to test: if you break a function wrong, then you might end up with a group of functions that only work together anyway.
For example: when you break a big function into 3 smaller ones. If the first acquires a resource (transaction, file) and the third releases it, then it might be simpler to test the whole group rather than each one separately.
Breaking an 80 line function into to 8x 10 line functions does not necessarily make it easier to test. Most of the time it just adds unit testing busy work, for no clear benefit. This becomes more clear if you imagine you wanted to test every possible input. Splitting the function in 8ths introduces roughly 8x the work, if each new function has the same number of possible input states. The math is more complicated in the general case, so you have to evaluate it on a case-by-case basis. Also, if you're trying to isolate a known bug, it might be beneficial to split the function and test each part in isolation.
Depends on the language. In general I find the way many unit tests are written to be very brittle. There is a balance here. If the 80 lines are clear and easy to understand they will likely be easy to test also. It’s very situational though. An 80 line function isn’t that bad. Check out the SQLite code base, which is extremely well tested, or the linux kernel. C code tends to push out the line count. Whereas 80 in Python is probably a bit much. Some libraries, especially GUI code tend to take a lot of lines, mostly just handling events and laying things out and there you often see big functions as well.
It might even make it harder to test: if you break a function wrong, then you might end up with a group of functions that only work together anyway.
For example: when you break a big function into 3 smaller ones. If the first acquires a resource (transaction, file) and the third releases it, then it might be simpler to test the whole group rather than each one separately.