fix: Add classpath elements to Surefire plugin

[Ansku]

Description

Maven doesn't use .classpath and as such isn't aware that it should use flow-server as a source directory. That leads to a whole lot of Project Errors in IntelliJ IDEA if flow-client module isn't ignored. This configuration change gets rid of those errors.

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.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[tepi]

I can't reproduce the original issue without this change (IDEA 2024.3 & latest macos). On the other hand this change does not seem to break anything for me either.

[mcollovati]

I couldn't reproduce the problem either.
Honestly, I wonder how adding a source folder to the classpath elements, where compiled classes are supposed to be added, can fix project errors.
However, since the flow-client setup for 2.11 is a bit tricky and the change does not seem to break anything as @tepi said, if it solves the issues for @Ansku I agree to merge.

[vaadin-bot]

This ticket/PR has been released with Vaadin 14.12.5.