fix: QDialog is not mounted in tests when used with mount() (fix #72)

[OneFourFree]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] New test runner
• [x] Documentation
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

If you are adding a new test runner, have you...? (check all)

• [ ] Created an issue first?
• [ ] Registered it in /packages/base/runners.json?
• [ ] Added it to /README.md?
• [ ] Included one test that runs baseline.spec.vue?
• [ ] Added and updated documentation?
• [ ] Included a recipe folder with properly building quasar project?

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] It's submitted to the dev branch and not the master branch
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• [x] It's been tested on Windows
• [x] It's been tested on Linux
• [x] It's been tested on MacOS
• [x] Any necessary documentation has been added or updated in the docs (for faster update click on "Suggest an edit on GitHub" at bottom of page) or explained in the PR's description.

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[IlCallo]

Seems fine, thanks for contributing!

[IlCallo]

Released as 3.0.0-alpha.9 :D