Add metadata to last solution cell to run hidden test cases if question has no public tests

[chrispyles]

If a question has no public test cases, otter assign does not include a check cell. This results in these tests being run at the end of the notebook and in no guaranteed order. This is bad. Instead, if there are no public test cases, otter should add metadata to the last cell of the question to ensure that the hidden test cases are run at the correct point during notebook execution.

# for example
cell["metadata"]["otter"]["tests"] = ["q1"]