Makes the maven plugin to generate Model-tests and API-tests in generated-test-sources

Created by: feech

The change makes the maven plugin to generate Model-tests and API-tests in generated-test-sources without changes for other generator.

Default behaviour for java generator is changing for the case when 'output' is not set.

Before, all model, api classes and tests for them went to 'target/generated-sources' Now, model and api classes go to the same folder - 'target/generated-sources', but tests for them go to 'generated-test-sources'.

There is a new additional option for java generator - 'testOutput'to set desired location for test-classes.

If output is set and testOutput is not - generator puts all files in output directory (this will keep backward compatibility) If testOutput is set the value overrides default behaviour for test files

closes #10917 (closed)

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 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  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*. For Windows users, please run the script in Git BASH.
• [+] File the PR against the correct branch: (master) 5.3.0, 6.0.x
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