Merge #12917: qa: Windows fixups for functional tests

Summary:
PR12917 backport.

fab9095d40 qa: Windows fixups for functional tests (MarcoFalke)

Pull request description:

Just two minor fixups to have less errors when the tests run on native windows.
* Strip whitespace from lines when reading from a notification file
* Instead of clumsily creating a file with weird permissions, just create a folder for the same effect in `mempool_persist.py`

Test Plan: ninja check-all

Reviewers: jasonbcox, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5169