Formula-Cookbook: Improve testing suggestions

Based on recent feedback on some pull requests, I touched up the section about adding tests to formula.
I wanted to make it clearer that the examples demonstrate how to create files on the fly and add a pointer to the tinyxml2 formula (DomT4 pointed it out to me as a good example).

Closes Homebrew/homebrew#49641.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>