[tests] add test_address method to wallet_import.py

Summary:
Adds a new test_address method for testing the
imported addresses.

Partial backport of Core PR14886
https://github.com/bitcoin/bitcoin/pull/14886/commits/fbdba40594d549c76aaf15401fe994ac5cfc4f73

Depends on D6087

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/D6088