test: Fix wallet_listdescriptors.py if bdb is not compiled

[hebasto]

If build system is configured --without-bdb, the wallet_listdescriptors.py fails:

$ test/functional/wallet_listdescriptors.py --descriptors
2021-07-14T13:20:52.931000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_02p7o1c9
2021-07-14T13:21:23.377000Z TestFramework (INFO): Test that the command is not available for legacy wallets.
2021-07-14T13:21:23.381000Z TestFramework (ERROR): JSONRPC error
Traceback (most recent call last):
  File "/home/hebasto/GitHub/bitcoin/test/functional/test_framework/test_framework.py", line 128, in main
    self.run_test()
  File "test/functional/wallet_listdescriptors.py", line 34, in run_test
    node.createwallet(wallet_name='w1', descriptors=False)
  File "/home/hebasto/GitHub/bitcoin/test/functional/test_framework/test_node.py", line 685, in createwallet
    return self.__getattr__('createwallet')(wallet_name, disable_private_keys, blank, passphrase, avoid_reuse, descriptors, load_on_startup, external_signer)
  File "/home/hebasto/GitHub/bitcoin/test/functional/test_framework/coverage.py", line 47, in __call__
    return_val = self.auth_service_proxy_instance.__call__(*args, **kwargs)
  File "/home/hebasto/GitHub/bitcoin/test/functional/test_framework/authproxy.py", line 146, in __call__
    raise JSONRPCException(response['error'], status)
test_framework.authproxy.JSONRPCException: Compiled without bdb support (required for legacy wallets) (-4)
2021-07-14T13:21:23.436000Z TestFramework (INFO): Stopping nodes
2021-07-14T13:21:24.092000Z TestFramework (WARNING): Not cleaning up dir /tmp/bitcoin_func_test_02p7o1c9
2021-07-14T13:21:24.092000Z TestFramework (ERROR): Test failed. Test logging available at /tmp/bitcoin_func_test_02p7o1c9/test_framework.log
2021-07-14T13:21:24.092000Z TestFramework (ERROR): 
2021-07-14T13:21:24.092000Z TestFramework (ERROR): Hint: Call /home/hebasto/GitHub/bitcoin/test/functional/combine_logs.py '/tmp/bitcoin_func_test_02p7o1c9' to consolidate all logs
2021-07-14T13:21:24.092000Z TestFramework (ERROR): 
2021-07-14T13:21:24.092000Z TestFramework (ERROR): If this failure happened unexpectedly or intermittently, please file a bug and provide a link or upload of the combined log.
2021-07-14T13:21:24.092000Z TestFramework (ERROR): https://github.com/bitcoin/bitcoin/issues
2021-07-14T13:21:24.092000Z TestFramework (ERROR): 

This PR fixes this issue.

Also see #20267.

[hebasto]

cc @S3RK

[achow101]

ACK 0c845e3