Fix race condition in SlowCoroWatchingThread #2318

Job	Run time
changes	7s
gatekeeper	4s
env	1s
coverage / generate and upload	4m 7s
scripttest / run	1m 45s
guitest / run (windows-latest)	3m 48s
guitest_nix / run (ubuntu-latest)	3m 4s
mac / build	9m 44s
pytest / run (windows-latest)	8m 9s
pytest / run (macos-latest)	8m 19s
guitest_nix / run (macos-latest)	3m 36s
ubuntu / build	9m 26s
windows / build	6m 2s
documentation / build	57s
	59m 9s

Provide feedback