refactor(fracmanager): reorganize test structure

[eguguchkin]

Fixes #245

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[forshev]

nit: some tests here make use of this pattern where docs are added to the last frac in actives slice and then new empty frac appended to actives, so fracs get filled the next iteration of a loop

maybe change it to more straightforward approach, which i find easier to understand, where new active frac created and filled with docs at the same time, like in TestReplayWithEmptyActive:

actives := make([]*frac.Active, 0, fracCount)
	for i := 0; i < fracCount; i++ {
		active := fp.CreateActive()
		appendDocs(t, active, 500+rand.Intn(100))
		actives = append(actives, active)
	}
	actives = append(actives, fp.CreateActive()) // last active frac is now empty

what do you think?