[tests] add test_importmulti method to wallet_import.py

Summary:
Adds a new test_importmulti method for testing the
importmulti RPC method.

Partial backport of Core PR14886
https://github.com/bitcoin/bitcoin/pull/14886/commits/fd3a02c381ee136e54579b5a2e3466c016101515

Depends on D6086

Test Plan:

./test_runner.py wallet_importmulti

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6087