test_registry.py: repeated numbers

### Issue Type

Code Clean-up

### Issue Description

During the review of #507, @cacraigucar originally wrote:

> Perhaps to keep these in sync in the future, we could have a variable that is set to the 4 on both these lines. But, I see this pattern is used throughout this file, so perhaps not worth changing.

This refers to the assertion error messages here
https://github.com/ESCOMP/CAM-SIMA/blob/e62a76359e49736eb236681ec83befdeced1ffa2/test/unit/python/test_registry.py#L108-L109

https://github.com/ESCOMP/CAM-SIMA/blob/e62a76359e49736eb236681ec83befdeced1ffa2/test/unit/python/test_registry.py#L136-L137

which duplicate what the code states. It would be annoying to write these as parameters, but this also causes drift if not.

### Will this change answers?

No

### Will you be implementing this yourself?

Any Software Engineer can do this

Open test/unit/python/test_registry.py and inspect the assertion error messages around lines 108-109 and 136-137, then review the surrounding repeated-number pattern. Run the relevant registry tests first; done means the duplicated values stay synchronized without changing the test answers.

Written by the indexing model from the issue text.