test: add unit test for redundant payment (#4860)

If the payee and payer are the same account, then the transaction fails
in preflight with temREDUNDANT.

src/test/app/TxQ_test.cpp

@@ -1036,6 +1036,9 @@ public:
         // Fail in preflight
         env(pay(alice, bob, XRP(-1000)), ter(temBAD_AMOUNT));
 
+        // Fail in preflight
+        env(pay(alice, alice, XRP(100)), ter(temREDUNDANT));
+
         // Fail in preclaim
         env(noop(alice), fee(XRP(100000)), ter(terINSUF_FEE_B));
     }