Import scripts from importwallet RPC

Summary:
This is the follow-up of D2416. The scripts are now imported using
importwallet.
The test is updated to check the import.

Partial backport of core PR11667
commits ef0c730, 68c1e00, part of 1bab9b2 and 656fde5.

Depends on D2416

Test Plan:

./test/functional/test_runner.py wallet_dump

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2417