test_files: reorganize the Creating Test Files section.

- Mention `duckpan new`
- Improve discussion of copying
- Present both methods (duckpan, copying) earlier

In some later commit, I will try to figure out how to actually present
`duckpan new` at the start of their project.  It doesn't work well in
the tutorial since that  builds from scratch.  Maybe a QuickStart guide?