update sample tests, fix Java tests

[martin-mfg]

• Running ./bin/generate-samples.sh does not update generated test files, because "Test files never overwrite an existing file of the same name." Therefore many test files in the samples directory were outdated. I temporarily changed the OpenAPI Generator code to allow overwriting test files and then ran ./bin/generate-samples.sh.
  (@wing328 As a permanent solution for this problem, we could introduce a new setting which allows test files to be overwritten. If you think this is a good idea, please let me know. I would then create a PR for this.)
• There were some custom, not-automatically-generated tests in the samples directory which were not listed in test_file_list.yaml. For these files I reverted the above changes.
  For some languages I wasn't sure what needs to be reverted. In case of doubt I reverted all overwritten files of the language or of a given sample project. For Java only, I added the reverted files to test_file_list.yaml.
• I added samples/client/petstore/java/okhttp-gson-dynamicOperations to the CircleCI pipeline, because it contained many custom tests, but wasn't run in any pipeline.
• I replaced the outdated forkMode setting in Java generators. (more info on StackOverflow)
• I removed junit-platform-runner in cases where it is not necessary anymore. If the junit-platform-runner dependency was present and we were using maven-surefire-plugin version 3, no tests were being run.
• The last three points led to several red pipelines due to failing Java tests. I fixed these Java tests.

I have split the changes into several commits to make them as comprehensible as possible. It might be helpful to review the commits one by one.

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
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)

[wing328]

quick went through changes to 2000+ files and didn't spot any issue

thanks for the PR to clean up the outdated test files.