Feature Request: Tutorial on using 1am (or simpler testing patterns)

Nobody has claimed this yet.

[1am](https://github.com/lmj/1am) is a classic CL testing framework that fits on a page.

`1am` is also a good testing framework for beginners because it is easy to learn for small projects.

I'd like to see a tutorial using it.

Here are some draft ideas for relevant content from my notes in discussion with [phoe](https://phoe.github.io/):

```lisp
(ql:quickload :1am)
(ql:quickload :alexandria)

(add-package-local-nickname :a :alexandria)

(defun my-length (l)
"Takes the length of a list or vector."
(etypecase l
(list (list-length l))
(vector (array-dimension l 0))))

(defun test-my-length ()
"Testing my-length with 1am."
(1am:is (= 12 (my-length (a:iota 12))))
(1am:is (= 12 (my-length (coerce (a:iota 12) 'vector))))

;; Check that passing a keyword signals an error.
(1am:signals type-error (= :hahaha (my-length (coerce (a:iota 12) 'vector))))
(1am:signals type-error (= :hahaha (my-length (a:iota 12)))))

(test-my-length)
```

Start by reviewing the issue's draft Common Lisp example and the linked 1am project, then run the supplied Quicklisp setup and tests. Done means a tutorial explains beginner testing with 1am, including the my-length examples and error checks, in a form suitable for the cookbook.

Written by the indexing model from the issue text.