[WIP] Ledger test experiments

Factor out TestContext …
* Use rich struct for TestAccount
* Make helpers return TXResult
  * implements bool() operator, for expect fun
  * has LedgerEntrySet, uncleared
  * has TER, applied members
  * has snapshot of ledger prior to application
* Use the new ripple::TestSuite with expectEquals
Focus not on the MACROS, which are arguably useful for TDD, but on the substance of the changes, crude as they are. Instead of always asserting success, for each transaction, we return a (future extendable) struct, with ledger snapshots, access to the LedgerEntrySet etc etc.