borheimdall: add test for span persistence

[taratorio]

1. Adds an eth/stagedsync/test package which provides a test Harness object
2. Adds the first automated test to the bor-heimdall stage regarding span persistence (more to come in subsequent PRs)
3. Fixes a bug in the bor-heimdall stage which was uncovered with the test - we do not fetch span 0 when we sync straight from blockNum=0 without snapshots
4. Reorganises all mocks to be placed under ./mock sub-package within their respective packages