Fix off-by-one errors in use of IsFinalTx()

Previously CreateNewBlock() didn't take into account the fact that
IsFinalTx() without any arguments tests if the transaction is considered
final in the *current* block, when both those functions really needed to
know if the transaction would be final in the *next* block.

Additionally the UI had a similar misunderstanding.

Also adds some basic tests to check that CreateNewBlock() is in fact
mining nLockTime-using transactions correctly.

Thanks to Wladimir J. van der Laan for rebase.

src/qt/transactionrecord.cpp

@@ -168,12 +168,12 @@ void TransactionRecord::updateStatus(const CWalletTx &wtx)
     status.depth = wtx.GetDepthInMainChain();
     status.cur_num_blocks = chainActive.Height();
 
-    if (!IsFinalTx(wtx))
+    if (!IsFinalTx(wtx, chainActive.Height() + 1))
     {
         if (wtx.nLockTime < LOCKTIME_THRESHOLD)
         {
             status.status = TransactionStatus::OpenUntilBlock;
-            status.open_for = wtx.nLockTime - chainActive.Height() + 1;
+            status.open_for = wtx.nLockTime - chainActive.Height();
         }
         else
         {