This now prints nested form fields correctly.

* Needed a separate string format for the container field.
* Had to use `consp`, otherwise `nil` checkbox values wouldn't be
  recognised and a corresponding `<value/>` line wouldn't be printed.
* Needed to add a missing nested field to the test's expected value.

Checkboxes don't work on IRS tax forms using the current flat XFDF
structure. In order to activate them, their values must be set using a
nested, or hierarchical field structure.

Add a test to facilitate development of nested XFDF field generation.

Originally took this idea from Steve Losh's cl-digraph:
https://github.com/sjl/cl-digraph/blob/936a18cb7de3a7c7e59c244cf4326f217afec85f/test/tests.lisp

However, it turns out it wasn't necessary, as I can reference the 1am
`run` function directly in the ASDF `test-op`.

Write `<field>` elements to the XFDF XML based on the cons cells in the
`fields` input list.

Change true and nil values to the FDF/XFDF checkbox value format of
"Yes" and "Off".

Also decided to use lowercase `t` to match the case of `nil`. Not sure
if that makes sense.

Write a test for the `write-xfdf` function.

Worked out how to create the test system from:

- https://lispcookbook.github.io/cl-cookbook/testing.html
- https://github.com/sjl/cl-digraph/blob/master/cl-digraph.test.asd
- https://github.com/fukamachi/dexador/blob/master/dexador-test.asd

Add the test string to the `write-xfdf` function to pass the test and
check that the test runner is set up correctly.