> Does reading the string b'I wish to buy a fish license.\n' not cause a problem, too?

This string TEST_STRING_1 is used for a single os.write() call, whereas TEST_STRING_2 is splitted and written in two parts with two os.write() calls.

I prefer minimal change, since I don't know well the pty module.

> Is reading len(expected) bytes the correct behavior for systems where normalize_output is needed?

Yeah, I looked at this function. normalize_output() can return a string shorter than the input: len(normalize_output(s2)) <= len(s2).

So I think that len(s2) < len(expected) is correct.