wally: update libwally to v1.5.6

This got fixed in libwally v1.5.2: 7e483c049b

```
    def test_sign_signed_psbt(node_factory, bitcoind, chainparams):
        l1 = node_factory.get_node()
        l1.fundwallet(10**6)

        psbt = l1.rpc.txprepare([{l1.rpc.newaddr('bech32')['bech32']: 10000}])['psbt']
        signed_psbt = l1.rpc.signpsbt(psbt)['signed_psbt']

        if TEST_NETWORK != 'liquid-regtest':
            # FIXME: ideally this would succeed, as a noop.  But it shouldn't crash
>           with pytest.raises(RpcError):
                 ^^^^^^^^^^^^^^^^^^^^^^^
E           Failed: DID NOT RAISE <class 'pyln.client.lightning.RpcError'>
```

Changelog-Changed: Update libwally to v1.5.6
Changelog-Fixed: JSON-RPC: `signpsbt` on an already-signed PSBT with taproot paths now succeeds as a no-op instead of failing.

external/libwally-core

@@ -1 +1 @@
-Subproject commit 12f5ac4ccf0e24df90f764db4c516a7ab7b74ad3
+Subproject commit 0c41f38fb1c201786e9c3ac9eae4f5f80c051399

tests/test_wallet.py

@@ -983,13 +983,12 @@ def test_sign_signed_psbt(node_factory, bitcoind, chainparams):
     psbt = l1.rpc.txprepare([{l1.rpc.newaddr('bech32')['bech32']: 10000}])['psbt']
     signed_psbt = l1.rpc.signpsbt(psbt)['signed_psbt']
 
-    if TEST_NETWORK != 'liquid-regtest':
-        # FIXME: ideally this would succeed, as a noop.  But it shouldn't crash
-        with pytest.raises(RpcError):
-            l1.rpc.signpsbt(signed_psbt)['signed_psbt']
-    else:
-        # Non-taproot works fine.
-        assert l1.rpc.signpsbt(signed_psbt)['signed_psbt'] == signed_psbt
+    # libwally >= 1.5.2 ignores duplicate taproot keypath entries
+    # (issue: ElementsProject/libwally-core#509,
+    # commit with fix: ElementsProject/libwally-core#7e483c049b0a4405801f010e60c9f0335d2a617f),
+    # so re-signing an already signed PSBT input is now a noop
+    # rather than an error, on both bitcoin and liquid.
+    assert l1.rpc.signpsbt(signed_psbt)['signed_psbt'] == signed_psbt
 
 
 def test_psbt_version(node_factory, bitcoind, chainparams):