Add mocking of input() to advanced testing.

[PythonCHB]

We have instroduced testing with mailroom() -- but it's hard to test user interaction.

But input() could be mocked or monkey-patched --maybe introduce that in advanced testing?

Or make it an Exercise?

[PythonCHB]

This was done for this winter's live class:

modules/Testing_advanced.rst

and mocking mailroom as an assignment.