feat: separation of unit tests into junit4/junit5 (#1363)

[MarcinVaadin]

Description

Separation of vaadin-testbench-unit into three modules:

• vaadin-testbench-unit-shared (common parts, no direct junit dependency)
• vaadin-testbench-unit (junit4)
• vaadin-testbench-unit-junit5 (junit5)

Fixes #1363

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.