feature #1775 add support for lit renderers to grid tester

[joelpop]

Description

LitRenderers in Grids can contain properties and functions, but have no way for them to be inspected or invoked, respectively.

This enhancement provides methods for the GridTester to support this capability.

Fixes #1775

Type of change

• [ ] Bugfix
• [x] Feature

Checklist

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

Additional for Feature type of change

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