Initiative: Upstream end-to-end testing

In order to ensure that we have a good test coverage, we need to create a test matrix and this is not a secret ingredient, so we should share it upstream.

Every time a new feature is added or a bug is fixed, we should revisit the matrix to ensure that a test covers it. Or explain why no test is required.

This shouldn't be just a list of tests. We should have the history for each test, so that we can display the current state, but also the trend. This would allow us to understand if the code covered by a given test is subject to regressions over time.