Make our test suite survive changes in the surrounding environment

This is a placeholder ticket to track what’s part of the 2019-2020 roadmap on this front.

We don’t know yet what we’ll have to do among:

• just fix the Sikuli situation
• replace Sikuli but keep everything else
• switch frameworks entirely (OpenQA?)

We’ll know better around January after we make more progress on #15460.

Related issues

• Related to #17031
• Blocked by #14585 (closed)
• Blocked by #12185 (closed)
• Blocked by #15460
• Blocked by #15870
• Blocks #16209
• Blocked by #16976 (closed)

Original created by @intrigeri on 15953 (Redmine)