[PoC] Run parallel E2E tests

[victorgcramos]

Running e2e tests takes a very long time. If we add more tests, more time.
It doesn't sound like a good option for us in terms of scalability...

Fortunately, sorry-cypress provides us a
good solution for avoiding the necessity to have a
Cypress Dashboard account.

The parallel run can't be used without the implementation of #2624, because
the way our tests are currently designed, calling the politeiawww api with two
parallel tests would trigger lots of user identities errors, making our tests fail.

For now, the parallel tests are supposed to be used on local development env
only, since it's an experimental feature.

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(Experimental) Run tests in parallel

Note: currently on experimental mode. Use it only for local development to
speed up your tests run.

You need to have Docker running in order to make this parallel testing

You can run parallel tests using the sorry-cypress
alternative, instead of regular Cypress in order to avoid using Cypress
Dashboard, which requires an account and other KYC stuff. Fortunately, this
alternative wraps cypress into one docker image and opens it for usage.

Parallel tests will only work with Mocked APIs tests suites

1. Setup your sorry-cypress docker image: yarn && yarn test:e2e:parallel:setup
2. Run the application on development mode: yarn start
3. Open a new terminal windows (yes, plural) and run yarn cy:run:parallel on
   each.

[victorgcramos]

We should be able to add that on the new architecture, once #2803 gets merged. We aren't using the user layer on the new app, so we should be able to run e2e in parallel without any issues.