add some more clarity to trigrams new unit tests section

[natasha-aleksandrova]

for test_get_random_follower_not_there
Note that there are a number of options here as to what to do – but make sure it returns something.

can it be clarified or suggested what they should return? it isn't obvious to students with less coding background.

also, make_sentence I think needs a bit more description as well. (based on feedback from student)

Trigrams can be a tough (and frustrating) assignment for some students.