test: add executable flag for wallet_watchonly.py

Summary: Backport of Core PR16742

Test Plan:

ls -l ../test/functional/*py
../test/functional/wallet_watchonly.py --configfile=./test/config.ini

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7909