Improve structure and fixture obtention for unit tests

[2opremio]

Since we moved away from the DB scenarios, the legibility and structure of unit tests has considerably degraded. This is particularly obvious in the db2/history/, actions and ingest/processors packages.

Some of the problems are:

• We are mixing suites and normal tests
• Fixtures are generated in a custom manner. Many tests have different approaches to generating them and there seem to be duplicated efforts across the tests. Good examples of this are effects_processor_test.go and transaction_operation_wrapper_test.go
• There isn't a good alternative to the DB scenarios. There are are no libraries/helpers to easily inject fixture data into the database. This has led to a decrement of coverage in db2/history (see the history_claimable_balance* and history_liquidity_pool* code, for instance.

@bartekn suggests going through the test structure and designing a helper library, like we have for integration tests (e.g. for integration tests we have common functions to create accounts etc ... )