[New Feature] hide hiddentest cells OR text_stub for clearhiddentests

[AnnabellBrocker]

Currently autograded test cells, which only consits out of hidden tests, is displayed to the students as an empty code cell:

"Keep in mind that wrapping all tests (for an “Autograder tests” cell) in this special syntax will remove all these tests in the release version and the students will only see a blank cell. It is recommended to have at least one or more visible tests, or a comment in the cell for the students to see."

Sometimes it would be better, if this cell would be hidden, as students might be confused by empty code cells. This is possible by setting the metadata of a code cell (codeCellTypeSelect.hide()). If this is not possible, it would at least be good to be able to insert a phrase by default for cells that only consist of hidden tests, such as text_stub in clearsolutions.

Is it possible to include this feature?