pytest: recreate wallet on bitcoind restart.

Doesn't seem to stick in master.  Andy Chow suggested we
simply turn off wallet on older versions, and always create/load.

```
[gw8] [ 40%] FAILED tests/test_misc.py::test_bitcoind_goes_backwards

============================================================= FAILURES ==============================================================
___________________________________________________ test_bitcoind_goes_backwards ____________________________________________________
[gw8] linux -- Python 3.8.5 /usr/bin/python3

node_factory = <pyln.testing.utils.NodeFactory object at 0x7f931859a760>
bitcoind = <pyln.testing.utils.BitcoinD object at 0x7f931865eee0>

    def test_bitcoind_goes_backwards(node_factory, bitcoind):
        """Check that we refuse to acknowledge bitcoind giving a shorter chain without explicit rescan"""
        l1 = node_factory.get_node(may_fail=True, allow_broken_log=True)

        bitcoind.generate_block(10)
        sync_blockheight(bitcoind, [l1])
        l1.stop()

        # Now shrink chain (invalidateblock leaves 'headers' field until restart)
        bitcoind.rpc.invalidateblock(bitcoind.rpc.getblockhash(105))
        # Restart without killing proxies
        bitcoind.rpc.stop()
        TailableProc.stop(bitcoind)
        bitcoind.start()

        # Will simply refuse to start.
        with pytest.raises(ValueError):
            l1.start()

        # Nor will it start with if we ask for a reindex of fewer blocks.
        l1.daemon.opts['rescan'] = 3

        with pytest.raises(ValueError):
            l1.start()

        # This will force it, however.
        l1.daemon.opts['rescan'] = -100
        l1.start()

        # Now mess with bitcoind at runtime.
>       bitcoind.generate_block(6)

tests/test_misc.py:1307:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
contrib/pyln-testing/pyln/testing/utils.py:399: in generate_block
    return self.rpc.generatetoaddress(numblocks, self.rpc.getnewaddress())
contrib/pyln-testing/pyln/testing/utils.py:322: in f
    return proxy._call(name, *args)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <bitcoin.rpc.RawProxy object at 0x7f93184f6a30>, service_name = 'getnewaddress', args = ()
postdata = '{"version": "1.1", "method": "getnewaddress", "params": [], "id": 1}'
headers = {'Authorization': b'Basic cnBjdXNlcjpycGNwYXNz', 'Content-type': 'application/json', 'Host': 'localhost', 'User-Agent': 'AuthServiceProxy/0.1'}
response = {'error': {'code': -18, 'message': 'No wallet is loaded. Load a wallet using loadwallet or create a new one with createwallet. (Note: A default wallet is no longer automatically created)'}, 'id': 1, 'result': None}
```

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -138,14 +138,6 @@ def bitcoind(directory, teardown_checks):
         raise ValueError("elementsd is too old. At least version 160000 (v0.16.0)"
                          " is needed, current version is {}".format(info['version']))
 
-    # Make sure we have a wallet, starting with 0.21 there is no default wallet
-    # anymore.
-    # FIXME: if we update the testsuite to use the upcoming 0.21 release we
-    # could switch to descriptor wallets and speed bitcoind operations
-    # consequently.
-    if not bitcoind.rpc.listwallets():
-        bitcoind.rpc.createwallet("lightningd-tests")
-
     info = bitcoind.rpc.getblockchaininfo()
     # Make sure we have some spendable funds
     if info['blocks'] < 101:

contrib/pyln-testing/pyln/testing/utils.py

@@ -1,5 +1,6 @@
 from bitcoin.core import COIN  # type: ignore
 from bitcoin.rpc import RawProxy as BitcoinProxy  # type: ignore
+from bitcoin.rpc import JSONRPCError
 from pyln.client import RpcError
 from pyln.testing.btcproxy import BitcoinRpcProxy
 from collections import OrderedDict
@@ -351,7 +352,7 @@ class BitcoinD(TailableProc):
             '-logtimestamps',
             '-nolisten',
             '-txindex',
-            '-wallet="test"',
+            '-nowallet',
             '-addresstype=bech32'
         ]
         # For up to and including 0.16.1, this needs to be in main section.
@@ -369,6 +370,10 @@ class BitcoinD(TailableProc):
         self.wait_for_log("Done loading", timeout=TIMEOUT)
 
         logging.info("BitcoinD started")
+        try:
+            self.rpc.createwallet("lightningd-tests")
+        except JSONRPCError:
+            self.rpc.loadwallet("lightningd-tests")
 
     def stop(self):
         for p in self.proxies:
@@ -461,6 +466,7 @@ class ElementsD(BitcoinD):
             '-server',
             '-logtimestamps',
             '-nolisten',
+            '-nowallet',
             '-validatepegin=0',
             '-con_blocksubsidy=5000000000',
         ]