[stockpile-docs] Testing lesson should reference actual Stockpile tests

[devgeeks]

@audreyso noticed:

the stockpile-repo has different tests. For instance, it has unit-tests: App.spec.js & stockAPI.spec.js while my version has About.spec.js, Services,spec.js, etc for unit-tests… should you mention that these tests are not valid since we changed code & will need to be updated? Or mention that these tests are just some example tests the user can use for reference?

We should either update the Testing lesson to actually create the tests for Stockpile or at the very least reference the actual Stockpile tests and have them refer to them an suggest they try and implement them.

[devgeeks]

Note: I am adding the actual Stockpile tests to the /docs/lesson12 folder for linking to either way.