e2e tests

[lajarre]

• A basic treasury.
• A more complex treasury.
• Spread.

As a first step, no need to have it run in CI, just build a script.
Ideally, the scripts should setup a backend with cache prepopulated so as to never rely on external providers.

We can think about running these tests on production after each deployment.

The obvious choice for running e2e tests scenarios is Cypress.