[tests] tidy up wallet_importmulti.py

Summary:
Fixes review comments from PR 14886.

This is a partial backport of Core PR15108 : https://github.com/bitcoin/bitcoin/pull/15108/commits/6be64ef02ca0832dfb74f74e5b026ead4cd7ef72

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6186