test: add coverage for -rpcwallet cli option

Summary:
and add -getinfo coverage with multiple wallets loaded.

> The bitcoin-cli -rpcwallet= option is an essential RPC/CLI option when more than one wallet is loaded (see bitcoin-cli -help | grep -A5 rpcwallet or src/bitcoin-cli.cpp::L61) and it currently has no test coverage.
>
> It is not only used by users, but also by the test framework and ~10 test files via get_wallet_rpc().
>
> This PR adds coverage, while simultaneously improving the -getinfo coverage when multiple wallets are loaded.

This is a backport of Core [[bitcoin/bitcoin#18724 | PR18724]]

Test Plan: `./test/functional/test_runner.py interface_bit*`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8975

test/functional/interface_bitcoin_cli.py

@@ -3,6 +3,7 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test bitcoin-cli"""
+from decimal import Decimal
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
@@ -93,9 +94,64 @@ def run_test(self):
             assert_equal(cli_get_info['paytxfee'], wallet_info['paytxfee'])
             assert_equal(cli_get_info['relayfee'], network_info['relayfee'])
             assert_equal(self.nodes[0].cli.getwalletinfo(), wallet_info)
+
+            # Setup to test -getinfo and -rpcwallet= with multiple wallets.
+            wallets = ['', 'Encrypted', 'secret']
+            amounts = [Decimal('59.99999550'), Decimal(9), Decimal(31)]
+            self.nodes[0].createwallet(wallet_name=wallets[1])
+            self.nodes[0].createwallet(wallet_name=wallets[2])
+            w1 = self.nodes[0].get_wallet_rpc(wallets[0])
+            w2 = self.nodes[0].get_wallet_rpc(wallets[1])
+            w3 = self.nodes[0].get_wallet_rpc(wallets[2])
+            w1.walletpassphrase(password, self.rpc_timeout)
+            w1.sendtoaddress(w2.getnewaddress(), amounts[1])
+            w1.sendtoaddress(w3.getnewaddress(), amounts[2])
+
+            # Mine a block to confirm; adds a block reward (50 BTC) to the
+            # default wallet.
+            self.nodes[0].generate(1)
+
+            self.log.info(
+                "Test -getinfo with multiple wallets loaded returns no balance")
+            assert_equal(set(self.nodes[0].listwallets()), set(wallets))
+            assert 'balance' not in self.nodes[0].cli(
+                '-getinfo').send_cli().keys()
+
+            self.log.info(
+                "Test -getinfo with multiple wallets and -rpcwallet returns specified wallet balance")
+            for i in range(len(wallets)):
+                cli_get_info = self.nodes[0].cli(
+                    '-getinfo').send_cli('-rpcwallet={}'.format(wallets[i]))
+                assert_equal(cli_get_info['balance'], amounts[i])
+
+            self.log.info(
+                "Test -getinfo with multiple wallets and -rpcwallet=non-existing-wallet returns no balance")
+            assert 'balance' not in self.nodes[0].cli(
+                '-getinfo').send_cli('-rpcwallet=does-not-exist').keys()
+
+            self.log.info(
+                "Test -getinfo after unloading all wallets except a non-default one returns its balance")
+            self.nodes[0].unloadwallet(wallets[0])
+            self.nodes[0].unloadwallet(wallets[2])
+            assert_equal(self.nodes[0].listwallets(), [wallets[1]])
+            assert_equal(
+                self.nodes[0].cli('-getinfo').send_cli()['balance'],
+                amounts[1])
+
+            self.log.info(
+                "Test -getinfo -rpcwallet=remaining-non-default-wallet returns its balance")
+            assert_equal(self.nodes[0].cli(
+                '-getinfo').send_cli('-rpcwallet={}'.format(wallets[1]))['balance'], amounts[1])
+
+            self.log.info(
+                "Test -getinfo with -rpcwallet=unloaded wallet returns no balance")
+            assert 'balance' not in self.nodes[0].cli(
+                '-getinfo').send_cli('-rpcwallet={}'.format(wallets[2])).keys()
         else:
             self.log.info(
                 "*** Wallet not compiled; cli getwalletinfo and -getinfo wallet tests skipped")
+            # maintain block parity with the wallet_compiled conditional branch
+            self.nodes[0].generate(1)
 
         self.stop_node(0)
 
@@ -114,7 +170,7 @@ def run_test(self):
                                     self.nodes[0].cli('getblockcount').echo)
         # Verify success using -rpcwait.
         assert_equal(
-            BLOCKS,
+            BLOCKS + 1,
             self.nodes[0].cli(
                 '-rpcwait',
                 'getblockcount').send_cli())