Added VirtualListTester

[joelpop]

Description

Added VirtualListTester to enable VirtualLists to be unit tested with TestBench.

As this tester and GridTester both support LitRenderers, the common code from GridTester was extracted to an interface HasLitRenderer so it could be shared by both testers.

Because a VirtualList is used to display a large list of rows, for its testing Instancio was added as a test dependency to the project to facilitate randomized generation of values.

Fixes #1721

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.