[tests] add get_multisig function to wallet_importmulti.py

Summary:
Adds a new get_multisig function which generates
a new multisig and returns the public keys,
private keys and all script and address types.

Backport of Core PR14886
https://github.com/bitcoin/bitcoin/pull/14886/commits/08a4a0f70f545e114bc35dc76056282cb04bf013

Depends on D6083

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/D6086