In #6441 we added support for updating over HTTPS but in some situations this will fail:

• Man-in-the-middle attack
• certificate misconfiguration on Piwik servers
• Murphy's law, i.e. a random reason like server/network misconfiguration, etc.

In every case except the first one the user has legitimate reasons to ignore the HTTPS error and update via HTTP.

To address that, the user will be shown this screen if downloading the archive over HTTPS fails:

Please note that if the system doesn't support HTTPS downloads, it will use HTTP automatically so this use case is not related to this PR.

Technical details:

• it will show the screen for any error while downloading over HTTPS, e.g. timeout or whatever. This is because the error messages depend on the method used (curl, fopen…) and the OS/PHP version so it's impossible to know exactly what went wrong. That's why we mustn't assume there is a MITM attack and don't be too alarming in the error message. Feedback on the wordings welcome.
• includes UI tests
• I moved existing CoreUpdater UI tests from tests/UI to the plugin folder
• to cover the scenario of HTTPS update with UI tests I had to do the following changes:
  • refactored the updater by extracting the code of the controller into a Updater class using dependency injection
  • added a way to provide DI configuration in test fixtures: I've extracted this commit into Test fixtures can set up DI config #7434 for easier code review and discussions.
  • for my use case, I mock the Updater and simulate that 1) there is a new version available, 2) updating over HTTPS fails, 3) updating over HTTP succeeds
• unfortunately the Updater itself is not tested but it wasn't before anyway. To test it I would have to mock many things so I'm not even sure it would be very meaningful. Anyway it's too hard today because there are too many things that are static/not using DI.
• unrelated but I improved the diffviewer a bit (pushed that to master)

FYI looking at the diff commit by commit would probably be easier to read. Also the first commit here belong to #7434, it's better to review it in the separate PR.

TODO before merging:

• merge Test fixtures can set up DI config #7434

TODO after merging:

• remove the screenshots that were moved in this plugin from the UI screenshots repository (1 and 2)