test: Fix intermittent failure in wallet_importmulti
The wallet is async, so after generating a block, we must call syncwithvalidationinterfacequeue. Otherwise the timestamp will be of the previous block.
getbestblockhash directly accesses the chainstate, so it is not needed there.
This is a backport of Core PR18753
Test Plan: ./test/functional/test_runner.py wallet_importmulti
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8991