Test improvements

[kuhnroyal]

• move some tests from http2/dio to shared test project
• add some new tests for validateStatus
• wrap SocketException in DioExceptionType.connectionError instead of DioExceptionType.unknown in the Http2Adapter -> this brings the behavior in line with the default adapters

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

[github-actions]

Code Coverage Report

Package	Base Coverage	New Coverage	Difference
dio	🟢 80.34%	🟢 81.09%	🟢 0.75%
Overall Coverage	🟢 80.34%	🟢 81.09%	🟢 0.75%

Minimum allowed coverage is 0%, this run produced 81.09%

[kuhnroyal]

We now have the comment working. Thanks to @tm1000 for merging and releasing clearlyip/code-coverage-report-action#50!

[AlexV525]

Wonderful, thanks for your great work @kuhnroyal @tm1000