Flaky test - extension-editors-bpmn.test.ts

Summary
The test `extension-editors-bpmn.test.ts` was failing frequently on the CI. I investigated this by running the tests on my local machine simultaneously with `stress` to stress the CPU, which helped me simulate lower compute power of CI machine.
Multiple reasons why this was happening were found:

• The resolution in which the tests are being run is quite low - 1024x768, while most of us use at least 1920x1080, so the tests are designed for the higher resolution. Even our Jenkins is being run at 1920x1080 (that's probably why the Jenkins was not failing so frequently).
• The default mocha timeout is only 2000ms, which often caused failure.
• The default timeouts of some tests were too low.
• The test "Opens UserTask.bpmn..." needed improvement (scroll element into view before asserting).

Goal
Stabilize the test `extension-editors-bpmn.test.ts`.

Acceptance criteria
The tests are stable and are not failing locally and neither on the CI.