Testing with Jest

[ricokahler]

Per our discussion here: #39 (comment)

We wanted to add tests with jest. I wanted to open this issue to track how we'll do that.

Here's a few questions:

Testing conventions

What conventions do you prefer?

1. it('blah', () => {}) or test('blah', () => {})?
2. do you want omit top-level describe or prefer to keep it?
3. file.spec.ts or file.test.ts?
4. What folder structure do you prefer? separate or inline?

Separate: a tests folder next to src at the root or

/gatsby-plugin-graphql-codegen
|-/src
  |-gatsby-node.ts
|-/tests
  |-gatsby-node.test.ts

Inline: a test file in the same src folder?

/gatsby-plugin-graphql-codegen
|-/src
  |-gatsby-node.ts
  |-gatsby-node.test.ts

I prefer inline test files because it makes me care more about the tests but I don't mind either convention.

CI and Coverage Reporting

Do you have any preference for tools? I've only really used travis and coveralls but idk maybe those tools are out of style? I can defer to you to set that up but it we're testing we might as well talk about CI and coverage reporting

[d4rekanguok]

Thanks @ricokahler, I don't mind any of these so please take the lead.

Most of the code in this repo is 'glue code', so I think end-to-end tests are more important, but I don't know how to write those kind of tests yet. Would it be a good idea to boot up a Gatsby site, then use Jest to compare the output snapshot does that make sense)?

I have had good experience with travis in the past!