tests: write the notification to different files to avoid race condition

Summary:
Backport of core PR14275.

This is slighly adapted to not reduce large fork coverage but allow the
test to pass on windows.

Test Plan:

./test/functional/test_runner.py feature_notifications

Ran on Linux, windows (depends on D5964) and OSX for sanity.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5966