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

Summary:
Backport of core [[https://github.com/bitcoin/bitcoin/pull/14275 | 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

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