Change UI regression test strategy from breadth-first to depth-first

Since the code is pretty much operating system agnostic, we could utilize depth-first test strategy for the UI regression test instead of depth-first.

• RTA can run the installation tests for all environments
• RTA can run all autotests from all folder for all environments
• But we could consider having dedicated Squish tests in <src>tests/UI folder for Windows and/or Linux environment
  • Developers can repeat those at any time in their own computers
  • We can get high automation check coverage against regression
  • Customers can use those tests as basis for their test assets
  • This can even be triggered frequently by other tools
    • e.g either Gerrit hook in the code review for the commit
    • QSR nightly test server (non-blocking CI)
    • Coin itself for a given configuration
  • We can even consider having integration tests for the Qt Design Studio
  • The tests can use the predefined capacity tests and generated QML, or that can be done in the runtime in the tests.
• And RTA can run these in detailed test for given configurations in a  case that frequent mandatory Qt Creator updates do not mess the test script integrity
  • Tests include installation tests that will always update the Qt Creator