Add a test which checks for proper finish of Bodies/Receipts on mutliple tries

[kamilchodola]

As per description in test:

!!!!! Test to be used when NonValidator mode is true. !!!!!

1. Wait for node to be synced (eth_syncing returns false)
2. Stop a execution node
3. Create a backup of database
4. Restart node but without NonValidator node flags
5. Wait until end of sync (eth_syncing returns false)
6. Check if logs "Fast blocks bodies task completed." and "Fast blocks receipts task completed." are there - those two should always been present meaning that tasks are properly finished.
7. Stop a node, restore backup, redo steps 5 and 6

WHY?
If there is no such logs, there is high chance we "lost" something and we should investigate what is missing 

[brbrr]

Besides small naming nitpick - LGTM

[brbrr]

Probably BackupDirectory is not the best name. Maybe CopyDirectory will work better?