DefaultCodegenTest use same test engine for all tests

[Mattias-Sehlstedt]

Changes so that the DefaultCodegenTest uses one test engine rather than each test using one of the two present in the project (junit and testng). This removes most warnings in the file that are not tied to a Raw use of parameterized class Schema warning.

These are fixes that I have done as part of an investigation into how to best extend useOneOfInterfaces to generated sealed interfaces. But since that might take a while I thought I might as well merge fixes that can be separated from that functionality.

Includes fix for samples that were not updated with this change.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
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  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
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[wing328]

tested locally and the result looks good

thanks for the contribution 👍