Lock example_status_persistence_file to prevent race conditions
Created by: Empact
This introduces file locking to ExampleStatusPersister#persist to avoid a race condition where parallel or unrelated spec runs race to update the same file:
- read 1
- read 2
- write 1
- write 2 - write 1 is lost
Because we need to read and write the file under the same lock, I inline #write into #persist so that dumped_statuses is called after the file is opened/locked.