sethdseed and importmulti are not available for descriptor wallets, so
when doing descriptor wallet tests, use importdescriptors instead.
Also changes some output to match what descriptor wallets will return.
When private keys are disabled, still fetch keys from the keypool
if the keypool has keys. Those keys come from importing them and
adding them to the keypool.
A blank wallet is a wallet that has no keys, script or watch only things.
A new wallet flag indicating that it is blank will be set when the wallet
is blank. Once it is no longer blank (a seed has been generated, keys or
scripts imported, etc), the flag will be unset.