[batch] Remove lint errors from test_batch and test_accounts

[daniel-goldstein]

Basically all naming, rids these test files of linting errors. We do a lot of reassigning a BatchBuilder variable to a Batch and so I consolidated around bb and b. A couple instances where I remove debug_info from an assert statement is because the associated Batch object would not exist, since that assert is triggered by an error that's raised before the Batch object is created.

[danking]

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