Added tester for RouterLink.

[joelpop]

Description

Adds missing ComponentTester for RouterLink. (Attempting to use AnchorTester in its place results in an error due to Anchor and RouterLink being different Components.

Tester methods include

• getHref
• getPath
• getQueryParameters
• getRoute
• click

Fixes #1805

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.