[tests] add get_key function to wallet_importmulti.py

Summary:
Adds a new get_key function which generates
a new key and returns the public key,
private key and all script and address types.

Partial backport of Core PR14886
https://github.com/bitcoin/bitcoin/pull/14886/commits/7c99614b40301daba5e526c6e3ab281965e869c6

Depends on D6051 and D6081

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/D6083