Add a test ensuring locale data is valid

[stephenmathieson]

Expectation: We should know that our locale files (./locales/*.json) are valid in order to prevent issues like #1127. This should be verified by a computer (CircleCI) on every release (or every commit?), not just done manually.

Actual: We do not test locale data.

Motivation: axe-core throws when given "bad" locale data.

An example test may look like:

const fs = require('fs')
const path = require('path')
const assert = require('assert')
const glob = require('glob')

const localeFiles = glob.sync('./locales/*.json')

describe('locales', () => {
  for (const localeFile of localeFiles) {
    describe(`locale ${localeFile}`, () => {
      it('should be valid', () => {
        const localeData = fs.readFileSync(path.join(__dirname, localeFile), 'utf-8')
        const locale = JSON.parse(localeData)
        assert.doesNotThrow(() => axe.configure({ locale }) )
      })
    })
  }
})