Feature: Sim mode at runtime #6814

Job	Run time
1/4	15m 30s
2/4	13m 35s
3/4	12m 15s
4/4	12m 10s
py-3-latest	20m 14s
flaky	7m 13s
_remote_background_indep_poll	5m 17s
_remote_background_indep_tcp	7m 11s
macos 1/5	19m 11s
macos 2/5	24m 13s
codecov	7s
	2h 16m 56s

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