Feature: Sim mode at runtime #6408

Job	Run time
1/4	14m 42s
2/4	20m 21s
3/4	15m 31s
4/4	13m 34s
py-3-latest	16m 50s
flaky	6m 15s
_remote_background_indep_poll	5m 5s
_remote_background_indep_tcp	9m 48s
macos 1/5	20m 17s
macos 2/5	37m 9s
codecov	0s
	2h 39m 32s

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