transaction_tests: Be more strict checking dust

* Don't allow off-by-one or more
* Make clear dust is coupled with minRelayTxFee
* Check rounding for odd values

src/primitives/transaction.h

@@ -143,8 +143,8 @@ public:
         // to spend something, then we consider it dust.
         // A typical spendable txout is 34 bytes big, and will
         // need a CTxIn of at least 148 bytes to spend:
-        // so dust is a spendable txout less than 546 satoshis
-        // with default minRelayTxFee.
+        // so dust is a spendable txout less than
+        // 546*minRelayTxFee/1000 (in satoshis)
         if (scriptPubKey.IsUnspendable())
             return 0;