ci: add workflow to test own action

[erisu]

Add a GitHub Action workflow to test itself

Example run: https://github.com/erisu/apache-rat-action/actions/runs/19169653238

This workflow tests the following use cases:

• Without .ratignore
  • Passes when all files have an ASF license header.
  • Fails when one or more files are missing the ASF license header.
• With .ratignore
  • Passes when all files have an ASF license header, excluding the files listed in .ratignore.

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Note

In the above example run, you will see an annotation showing an error from the “test without .ratignore” use case, which exits with code 1. This is expected because we are testing the scenario where a file is missing the license header, which correctly triggers an error. The workflow is configured to ignore errors during this test, and we later verify that the error was thrown and report the test as successful.