Add automated test workflow and pin example third-party actions to commit SHAs

[oto-macenauer-absa]

The task

Two related CI/security hygiene improvements:

1. Add .github/workflows/test.yml

The action has no automated tests. A test workflow should:

• Generate fixture certificates using openssl req covering: valid, expiring-soon, expired, not-yet-valid, missing file, and multi-cert with a valid replacement
• Run the action against each fixture set and assert correct exit codes and step-summary content
• Execute on both ubuntu-latest and macos-latest to catch date portability regressions

2. Pin actions/checkout to a full commit SHA in examples/validate_certificates.yml

The example currently uses the mutable @v4 tag, which is vulnerable to supply-chain attacks via tag mutation. Replace with a pinned SHA, e.g.:

uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2