refs https://linear.app/tryghost/issue/CORE-74/improve-the-test-situation
- this commit:
- adds the `ember-cli-code-coverage` dependency to collect
coverage from ember tests
- enables the `COVERAGE` env variable in CI
- merges coverage data because tests are run in parallel
- adds the codecov GitHub Action into CI so we can upload
coverage reports
no issue
- improves test run times by splitting the tests into multiple groups and running each in their own browser instance
- settled on 2 browser instances as that seems to best fit within Travis' memory and CPU constraints
- updated ember-cli-code-coverage config to work with parallel builds