test: Set up E2E tests

[pahund]

Complex functionality like lazy loading, asynchronous config loading or config switching should be tested through end-to-end tests.

A promising setup: Storybook combined with Cypress.

Run the E2E tests with Travis.

Measure code coverage with Istanbul, as explained in the Travis docs.

Combine the coverage data with the coverage data from the Jest unit tests and send the combined data to Coveralls.

As a first spike, set up a test that just renders a plain old GCP ad.