fix: GridTester.sortByColumn(): consider the value of MultiSortPriority as well.

[mvysny]

Description

GridTester.sortByColumn(): consider the value of MultiSortPriority as well.

Fixes #1695

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[CLAassistant]

All committers have signed the CLA.

[mvysny]

@mcollovati thank you for the approval 🙏 @caalador could you please merge? I'm not authorized to merge this PR.