test: add coverage for abandoning unconfirmed transaction

Co-authored-by: Eunovo <eunovo9@gmail.com>

Author: rkrux

test/functional/wallet_abandonconflict.py

@@ -45,6 +45,10 @@ def run_test(self):
         txB = alice.sendtoaddress(alice.getnewaddress(), Decimal("10"))
         txC = alice.sendtoaddress(alice.getnewaddress(), Decimal("10"))
         self.sync_mempools()
+
+        # Can not abandon transaction in mempool
+        assert_raises_rpc_error(-5, 'Transaction not eligible for abandonment', lambda: alice.abandontransaction(txid=txA))
+
         self.generate(self.nodes[1], 1)
 
         # Can not abandon non-wallet transaction