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
