Add a --descriptors option to various tests

Adds a --descriptors option globally to the test framework. This will
make the test create and use descriptor wallets. However some tests may
not work with this.

Some tests are modified to work with --descriptors and run with that
option in test_runer:
* wallet_basic.py
* wallet_encryption.py
* wallet_keypool.py
* wallet_keypool_topup.py
* wallet_labels.py
* wallet_avoidreuse.py

test/functional/wallet_labels.py

@@ -115,15 +115,16 @@ class WalletLabelsTest(BitcoinTestFramework):
             assert_raises_rpc_error(-11, "No addresses with label", node.getaddressesbylabel, "")
 
         # Check that addmultisigaddress can assign labels.
-        for label in labels:
-            addresses = []
-            for x in range(10):
-                addresses.append(node.getnewaddress())
-            multisig_address = node.addmultisigaddress(5, addresses, label.name)['address']
-            label.add_address(multisig_address)
-            label.purpose[multisig_address] = "send"
-            label.verify(node)
-        node.generate(101)
+        if not self.options.descriptors:
+            for label in labels:
+                addresses = []
+                for x in range(10):
+                    addresses.append(node.getnewaddress())
+                multisig_address = node.addmultisigaddress(5, addresses, label.name)['address']
+                label.add_address(multisig_address)
+                label.purpose[multisig_address] = "send"
+                label.verify(node)
+            node.generate(101)
 
         # Check that setlabel can change the label of an address from a
         # different label.