[google-auth-library-java] Document Creation and Debugging Process for Test Certificates

As requested in PR googleapis/google-auth-library-java#1746, we need to document the process for creating and handling the dummy certificates used in our tests. This will help future maintainers understand and troubleshoot test cases involving these certificates.

Tasks:

- Document the steps taken to generate the dummy certificates currently used in the test suite.

- Provide guidance or list tools/commands for decoding, parsing, and debugging these PEM-encoded certificates to help troubleshoot any flaky or failing tests in the future.

- Add this documentation to a DEVELOPMENT.md file in the repository.