Add test module for warning when no root is set

[OleksandrOrlov]

The PR is one in set of PRs to add test modules to check partitioning warnings by splitting partitioning_warnings test module.

The test module checks that Error Dialog is shown when saving
partitioning settings with no root mounted.

• Related ticket: https://progress.opensuse.org/issues/78007
• Verification run:
  sle: https://openqa.suse.de/tests/5141485
  TW: https://openqa.opensuse.org/tests/1499589

[sofiasyria]

I think it would be faster if we just removed the root partition, accept changes and then add it after error message. But as it's not taking too long as is, LGTM . Just check the Travis failure and maybe break the lines that are longer than 120 characters. Kindly take a look here: https://gitlab.suse.de/qsf-y/qa-sle-functional-y/-/blob/master/Development_Guide.md#self-review-check-list

[OleksandrOrlov]

I think it would be faster if we just removed the root partition, accept changes and then add it after error message.

I also considered the faster way to do that. As I noticed, if select "Start with existing partitions" then there is not root there at all, then I can just hit "Accept" and got the error. The potential problem that I see in that case is that it breaks down the independency of the test. If the existing proposal will be different than I expect (e.g. we'll add some test module before the one), then the result might be unexpected. Maybe the issue has low potential for appearing, but I just wanted to predict it.

[foursixnine]

@rwx788 signs with blood here.