Add assertion to test scripted examples

[speedytwenty]

The scripts that generate the cli-table3 docs include "testing" routines which never actually get tested. It's unclear whether this was lost or dropped somewhere along the way.

Here I've added a simple routine that tests the tables generated with the defined expectations.

It would be ideal if these "tests" were induced with the jest tests, but as is they are only induced within the "print-examples" and "update-docs" scripts.