Prepare TransactionManager tests for multi-repo support

TransactionManager currently only supports one repository. In order to support object pooling, we'll have to support multiple repositories in a partition in order to place the pool and the member repositories in the same partition. As a first step towards supporting that, this MR refactors the TransactionManager tests to support asserting the state of multiple repositories. This will be used to assert the state of all repositories in a storage at the end, and to also assert a given transaction's snapshot contains the expected repositories.