migrate test_reg to pytest

[frogunder]

What does this PR do?

migrate test_reg to pytest

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated

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Yes

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[dwoz]

@frogunder Can you please resolve the conflicts?

[dwoz]

@frogunder Unfortunately there are conflicts now. I'll merge this if you ping me when they are resolved.