[DO NOT MERGE] POC tests at component level

[radiocontrolled]

Resolves #193

Tests at the component level
This PR creates a psammead-test component, with Mocha and Chai installed as devDependencies in the component's package.json. The component has a test script, which runs a test (relevant doc):

  "scripts": {
    "test": "mocha --compilers js:babel-register"
  },

Another test script, which echoes to the console, is added to psammead-brand.

Testing at the parent level
In the parent package.json of Psammead, I added a "test:packages" script. It calls lerna run test which will look for a script of name 'test' in each package and will run it (see lerna run docs). I chose lerna run test over lerna exec -- npm test because it's cleaner - the later will return "Error: no test specified" for all components that lack a test script.

"scripts": {
    ...
    "test": "npm run test:lint && npm run test:lint:css && npm run test:unit && npm run test:packages",
    "test:packages": "lerna run test",
   ...
}

Reviewing this PR

1. Install Lerna globally if it is not installed
   npm install --g lerna
lerna --version

2. Do npm test

The usual linting and Jest unit tests will run. This will be followed by the test scripts from psammead-brand and psammead-test.

Implications
New components can be added, with their own testing devDependencies (e.g. mocha/chai in this POC). They might be hooked into the psammead level npm test script by being called a consistent name (e.g. "test") and invoked via lerna run test.

However, mandating a consistent script name for component-level tests could prove annoying. Let's say there are several Psammead components have bespoke component-level tests, psammead-component-A, psammead-component-B and psammead-component-C. It might be annoying for the author of psammead-component-C to run test scripts from other components if this adds significantly to testing time.

• I have assigned myself to this PR and the corresponding issues
• Tests added for new features
• Test engineer approval

[dr3]

👍 We would need to document somewhere that we auto run .test.js files using our testing suite, however you can do your own testing using a test folder or .spec.js files.

As a POC looks great, make sure this doesn't get merged or @bbc/psammead-test will publish though 🙈

[bcmn]

This looks great to me, nice & understandable. 👍

Definitely agree on Drew's point though, we'd have to document the Jest behaviour.

[radiocontrolled]

@dr3 @bcmn I agree. I think some of this documentation would belong in CONTRIBUTING.md (how to add custom tests) and maybe in the top level readme (explanation of current behaviour). Will solicit some more feedback on this PR.

The current Jest behaviour looks for scripts and packages that have test.js/test.jsx files. It also looks for a __tests__ folder

  "jest": {
    "collectCoverageFrom": [
      "**/packages/**/*.{js,jsx}",
      "scripts/**",
      ...
    ],
    "testMatch": [
      "**/__tests__/**/*.js?(x)",
      "**/*.test.{js,jsx}"
    ]
  },

[radiocontrolled]

This PR should not be merged.
It's closed as the outcome will be a PR to contributing.