Fixup paths in wallet_multiwallet

Summary: Partial backport of [[bitcoin/bitcoin#14559 | PR14559]] to better match Core

Test Plan:
```
test_runner.py wallet_multiwallet
```

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5546

test/functional/wallet_multiwallet.py

@@ -75,7 +75,7 @@ def wallet_file(name):
         extra_args = ['-wallet={}'.format(n) for n in wallet_names]
         self.start_node(0, extra_args)
         assert_equal(set(map(lambda w: w['name'], self.nodes[0].listwalletdir()[
-                     'wallets'])), set(['', 'w3', 'w2', 'sub/w5', 'w7', 'w7', 'w1', 'w8', 'w']))
+                     'wallets'])), set(['', 'w3', 'w2', os.path.join('sub', 'w5'), 'w7', 'w7', 'w1', 'w8', 'w']))
 
         assert_equal(set(node.listwallets()), set(wallet_names))
 
@@ -197,7 +197,7 @@ def wallet_file(name):
         self.restart_node(0, extra_args)
 
         assert_equal(set(map(lambda w: w['name'], self.nodes[0].listwalletdir()['wallets'])), set(
-            ['', 'w3', 'w2', 'sub/w5', 'w7', 'w7', 'w8_copy', 'w1', 'w8', 'w']))
+            ['', 'w3', 'w2', os.path.join('sub', 'w5'), 'w7', 'w7', 'w8_copy', 'w1', 'w8', 'w']))
 
         wallets = [wallet(w) for w in wallet_names]
         wallet_bad = wallet("bad")
@@ -365,7 +365,7 @@ def wallet_file(name):
         assert_equal(w1.getwalletinfo()['walletname'], 'w1')
 
         assert_equal(set(map(lambda w: w['name'], self.nodes[0].listwalletdir()['wallets'])), set(
-            ['', 'w3', 'w2', 'sub/w5', 'w7', 'w9', 'w7', 'w8_copy', 'w1', 'w8', 'w']))
+            ['', 'w3', 'w2', os.path.join('sub', 'w5'), 'w7', 'w9', 'w7', 'w8_copy', 'w1', 'w8', 'w']))
 
         # Test backing up and restoring wallets
         self.log.info("Test wallet backup")