Add test for multiwallet batch RPC calls

Tests bug reported in https://github.com/bitcoin/bitcoin/issues/11257

test/functional/multiwallet.py

@@ -76,5 +76,9 @@ class MultiWalletTest(BitcoinTestFramework):
         assert_equal(w2.getbalance(), 1)
         assert_equal(w3.getbalance(), 2)
 
+        batch = w1.batch([w1.getblockchaininfo.get_request(), w1.getwalletinfo.get_request()])
+        assert_equal(batch[0]["result"]["chain"], "regtest")
+        assert_equal(batch[1]["result"]["walletname"], "w1")
+
 if __name__ == '__main__':
     MultiWalletTest().main()