Clean up test utils, make helpers to avoid code duplication and remove confusing util functions

[shahzadlone]

Clean up test utils

Make assert helpers

To avoid code duplication that was introduced in #3608

Remove confusing utility functions

• Remove getNodes and getNodeCollections to avoid more bugs like #3608

Improve assertions

• https://github.com/sourcenetwork/defradb/issues/1356

Improve documentation

• For some test cases like backup export and import in https://github.com/sourcenetwork/defradb/issues/3067