feat(gatsby): update cache.set to resolve with set value

[mgmolisani]

Description

Cache set method will now resolve to the stored value. Both cache set and get now also reject on cache errors.

Also, I have never written Jest tests so this was a pretty exciting learning experience. Mocking was not very intuitive, especially due to the resolve function being in the callback of the get and set cache manager package. We also don't actually talk to the cache so the get tests feel a little wonky. Also, I do not know the shape of the error object sent from the cache manager or what actually can cause it so I mocked its existence with !undefined which is obviously just true but I wanted it to read as something that was defined and not just true. Is there a better/conventional way to say err is defined? Would {} be a better solution? Interested if anyone has a better testing strategy than just forcing the error argument to exist or not.

Related Issues

Addresses #11275

[mgmolisani]

General Jest question after continuing to think of Jest best practices. For the code where I need the err and res variables set dynamically per test, would it be better to declare regular variables and clean them up with an afterEach? Or if calling them as mocked functions like they are now, is it better practice to use jest.fn().mockReturnValue() when we are using them like variables and not functions since the body of the function is always just the returned value? Really deep diving into Jest but it doesn't seem like mocking variables is explained in the docs (likely because you can literally just use variables or callback mocked functions to return a value as stated above). What do other people think?

[DSchau]

@mgmolisani yep! Just update with a new commit (as many as you want actually!). Come merge time, we'll just squash all those commits down into one nice, clean commit message :)

[mgmolisani]

@DSchau Updated with suggestions (sorry if this is basically just a duplicate notification).

[wardpeet]

There are few things that should be changed

[mgmolisani]

Updated. Now resolves to undefined on errors. Is this the behavior that is desired?

(Also, as a git contributing noob, I didn't know that I should pull the official master branch into my topic branch to sync with the prod build. Caused a bunch of test failures. Is this something that is worth a pull request to the docs for?)

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