There are several places in core_accesslib_testcase where a new teacher is created and that teacher's id is passed instead of the expected role id. This probably happened because in different parts of the code $teacher can mean either a teacher role or the new teacher. The unit tests still pass because the tests use editingteacher (default role id is 3) and the generated teacher has an id of 3. There's no intrinsic reason for this to be the case, and if you're ever running a custom environment which starts with more than two users before this test begins then it will fail.