test: Fix intermittent failure in wallet_importmulti

Summary:

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