Drive the line-based UI in-process: input() answers come from a list
and the output is captured. The tests walk the main menu through
creating, playing, querying, deleting, importing and exporting
characters, plus bad menu input and the Ctrl+C handler.
The play loop runs on a fake clock that advances a minute per call and
stops when time.sleep has been called a set number of times, so it
levels up and saves on schedule without waiting.
The test helpers move to tests/helpers.py, which now also holds the
input() feeder.
