`TestBuilder_RestartSmeshing` fails when `-race` flag is enabled

[nkryuchkov]

Description

2022-01-19T20:40:16.1885250Z ?   	github.com/spacemeshos/go-spacemesh	[no test files]
2022-01-19T20:41:04.3316844Z --- FAIL: TestBuilder_RestartSmeshing (0.06s)
2022-01-19T20:41:04.3322460Z     activation_test.go:330: 
2022-01-19T20:41:04.3323133Z         	Error Trace:	activation_test.go:330
2022-01-19T20:41:04.3323679Z         	Error:      	Should be false
2022-01-19T20:41:04.3324239Z         	Test:       	TestBuilder_RestartSmeshing
2022-01-19T20:41:04.3324619Z FAIL
2022-01-19T20:41:04.3325256Z FAIL	github.com/spacemeshos/go-spacemesh/activation	37.968s

https://github.com/spacemeshos/go-spacemesh/runs/4873660080?check_suite_focus=true https://github.com/spacemeshos/go-spacemesh/pull/2930

[poszu]

@nkryuchkov To me, it seems that a data race does not cause failure. The race detector does not report the problem. I think the test is generally flaky because of the Sleep() in it and it manifests when ran with -race because it slows down a lot.