Adds test coverage for the wallet's crypted key loading from db process.
The following scenarios are covered:
1) "All ckeys checksums valid" test:
Loads an encrypted wallet with all the crypted keys with a valid checksum and
verifies that 'CWallet::Unlock' doesn't force an entire crypted keys re-write.
(we force a complete ckeys re-write if we find any missing crypted key checksum
during the wallet loading process)
2) "Missing checksum in one ckey" test:
Verifies that loading up a wallet with, at least one, 'ckey' with no checksum
triggers a complete re-write of the crypted keys.
3) "Invalid ckey checksum error" test:
Verifies that loading up a ckey with an invalid checksum stops the wallet loading
process with a corruption error.
4) "Invalid ckey pubkey error" test:
Verifies that loading up a ckey with an invalid pubkey stops the wallet loading
process with a corruption error.
