[TC] [Objstore] Improve function names in testify testing

What's the issue?
The unit testing in 'objstore' doesn't use useful function names for mocked functions. As an example, fn1.

What's the fix?
We should look to improve this. There's a couple of issues to consider:

1. Variable name shaddowing
2. Different typing for shaddowed variables

I believe this is why we chose to use this type of naming initially.