[tests] tidy up imports in wallet_importmulti.py

Summary:
Partial backport of Core PR14886
https://github.com/bitcoin/bitcoin/pull/14886/commits/e5a8ea8f146393e2a5e7d73a49335f3670bada11

The parts excluded are only used in the witness-related code.

Test Plan:

./test_runner.py wallet_importmulti

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6081