test_files: better explain the whys and hows

Tell developers :
- how test files help them
- what a good one should contain
- how and where to create the file
- how and when to run the tests

I went from a presumption that they aren't necessarily familiar with the
tooling or why testing is important.
test_files: improvements from review feedback.

- Deawkwardize first paragraph
- Change 'triggering properly' to 'functioning properly' and explain
- Better communicate what is being tested when creating a new file
- Expand testing descriptions to include `duckpan test`

Also includes some minor changes based on my own read through.
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?
test_files: rework "Creating Test Files" section a bit.

Based on the always appreciated feedback of @moollaza:

- Add elided words in edit description.
- Clarify naming and square up example.
- Explain whence come the testing libraries.
Merge pull request #46 from mwmiller/test_files

test_files: better explain the whys and hows