autotest: fix testing and simulated register scaling #41059

Job	Run time
build (base)	6m 29s
build (clang)	8m 20s
autotest (sitltest-rover)	18m 37s
autotest (sitltest-sailboat)	2m 50s
autotest (sitltest-balancebot)	8m 25s
	44m 41s

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