Use importdescriptors when in descriptor wallet mode in wallet_createwallet.py

Summary:
sethdseed and importmulti are not available for descriptor wallets, so when doing descriptor wallet tests, use importdescriptors instead

The keypool size when using descriptor wallets is unaffected for Bitcoin ABC, because our descriptors wallets have only the p2pkh keys.

This is a backport of core#18788 [7/15]
https://github.com/bitcoin/bitcoin/pull/18788/commits/25bc5dccbfd52691adca6edd418dd54290300c28

Test Plan: test/functional/test_runner.py wallet_createwallet

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10669