Add a new wb test for getRawSigs()

And a few extra improvements:

 * Simplify the existing TestWithdrawal, by moving some of its checks into
   assertion functions

 * Change a few test helpers that used to take both a pool and a manager
   argument to take just the pool, as the manager can be retrieved from there.

 * Rename a few things, like TxInSignatures to TxSigs as it really holds the
   sigs for all inputs in a tx

 * Fix TstCreatePkScript() to lock the manager after it's done -- callsites
   should explicitly unlock it if they need so