Transition all relevant testcases to it. In the process, port
docstrings/comments from test.py files to expected_concrete_syntax.lkt
(now test.lkt) sources and fix stylechecks issues there.
(cherry picked from commit e1c4fe5804)
By default, make App instances print parsing errors for all requested
units on the standard output, and let subclasses override this behavior
if needed.
Also extend the Python playground so that parsing errors fit nicely into
the prompt header.
