Auto-ignore lines calling the exception test function in Otter Assign

[chrispyles]

For exception tests in Otter Assign, test cells are formatted like

def test(var):
    # some logic

test(var) # IGNORE

Needing to remember the # IGNORE comment is cumbersome. Since each cell should have only 1 test in it, we can parse the cell and remove all lines outside the function body.

[chrispyles]

This can probably be done by parsing the cell's source as an AST, removing all nodes except the first def node, and using astunparse to convert this back to Python code.