Add solid test cases

[Fullstop000]

Increase the coverage of test cases over 90%.
See https://codecov.io/gh/Fullstop000/wickdb

Most essential parts:

• Iterator
• Record
• ShardeCache
• Memory Table
• Block
• Table
• DB
  • Basic test harness
  • Common read/write on mem table
  • Memtable compaction
  • SSTable compaction
  • DBIterator
  • Recovery
  • Snapshot
  • Manual Compaction
  • Simple fault injection based on MemStorage
  • Concurrency testing