Update RenderingIface_TEST's RegisterEngine test to only load the specified engine (backport #1098)

[mergify]

🦟 Bug fix

Fixes #1072

Summary

The RenderingIface_TEST tried to load the ogre (ogre 1.x) plugin even when GZ_ENGINE_TO_TEST set to ogre2. This PR updates the test to ensure that the specified engine is used in the test. While doing this I found and fixed a bug when unregistering a default engine by name (e.g. ogre2).

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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This is an automatic backport of pull request #1098 done by [Mergify](https://mergify.com).