fix: fallback to context classloader when loading class

[mcollovati]

Description

Currently classes are loaded with Class.forName, that uses the classloader from the calling class. However, this might not work on all environments because of different class loading setup (e.g in Quarkus testing). This change uses the current thread context class loader as fallback when Class.forName is not able to load a class.

Fixes vaadin/quarkus#139 Part of #1655

Type of change

• [x] Bugfix
• [ ] 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.
• [ ] 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.

[mcollovati]

Fix can be tested by running a UI Unit test on a Quarkus based application. Running MainViewTest should fail with a ClassNotFoundException, whereas it should pass when using the patched Testbench artifacts.