Apply predetermined block for testnet fork

PR #589 caused a condition for testnet difficulty to not match at
a couple of cases pre block 145k leading to issues with -reindex.

This makes the testnet fork hard at block 157500, while retaining
every case before that block.
Merge pull request #592 from patricklodder/1.8-dev-testnet-fork-fix

Apply predetermined block for testnet fork