test: check_mempool_result negative feerate

Adds test in mempool_accept to check if a negative maxfeerate is inputed
into check_mempool_result, asserts "Amount out of range" error message
and -3 error code

test/functional/mempool_accept.py

@@ -96,6 +96,12 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             rawtxs=[raw_tx_in_block],
             maxfeerate=1,
         ))
+        # Check negative feerate
+        assert_raises_rpc_error(-3, "Amount out of range", lambda: self.check_mempool_result(
+            result_expected=None,
+            rawtxs=[raw_tx_in_block],
+            maxfeerate=-0.01,
+        ))
         # ... 0.99 passes
         self.check_mempool_result(
             result_expected=[{'txid': txid_in_block, 'allowed': False, 'reject-reason': 'txn-already-known'}],