Java client codegen cleanup

[ronreynolds]

some code cleanup of JavaClientCodegenand a few minor behavior changes

• replacing many String.equals with boolean variables or isLibrary() in cases where the same library test isn't performed multiple times
• removed pointless boxing/unboxing of boolean
• some line-wrapping (much of it removed because there were too many changes in diff; perhaps a later MR...)
• removed possibly-moot edge-case where "okhttp-gson" and "" would have been treated slightly differently
• many assertEquals args in JavaClientCodegenTest are backwards (expected vs actual)
  • also this unit-test uses 3 different assertion frameworks (junit-jupiter, testng, and assertj)
  • junit-jupiter and testng both have assertEquals methods with expected and actual args swapped but identical behavior
  • converted a few junit-jupiter assertEquals to testng to make IntelliJ happy but there's so much more to do here in a future PR

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*.
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[wing328]

lgtm. thanks for enhancement 👍