Various backports in wallet.py

Summary:
Backport Core PR #10859: RPC: gettxout: Slightly improve doc and tests

6d2d2eb49 RPC: gettxout: Slightly improve doc and tests (Jorge Timón)

Backport Core PR #10257: [test] Add test for getmemoryinfo

d4668f3 [test] Add test for getmemoryinfo (Jimmy Song)

Backport Core PR #10256: [test] Add test for gettxout to wallet.py

dd1ea59 [test] Add gettxout call (Jimmy Song)

Test Plan:

./test/functional/test_runner.py wallet

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1162