Optimize / fix ScenarioConfigs.GetFullExecutionRequirements()

[na--]

While reviewing #2573, I noticed some strange code in ScenarioConfigs.GetFullExecutionRequirements(), specifically this part:

k6/lib/executors.go

Lines 318 to 321 in 556747f

	currentTimeOffset = step.TimeOffset
	currentPlannedVUs[step.configID] = step.PlannedVUs
	currentMaxUnplannedVUs[step.configID] = step.MaxUnplannedVUs
	addCurrentStepIfDifferent()

It seems that currentTimeOffset is meaningless and we will always add all intermediate steps to consolidatedSteps, which is not ideal. It's probably just a minor performance problem and not a consequential bug because GetFullExecutionRequirements() is not used for anything besides figuring out the max test VUs and duration at this point in time, but it could be a nasty surprise for us in the future if left unfixed.

Digging into the history of the code, it seems like it's like this because of a bugfix (#1362) for another issue (#1358) from before we even release executors in k6 v0.27.0 🤯 Before that the code made a bit more sense, even if it was buggy:

k6/lib/executors.go

Lines 311 to 316 in 55a49d5

	for _, step := range trackedSteps {
	// If the time offset is different, create a new step with the current values
	if step.TimeOffset != currentTimeOffset {
	addCurrentStepIfDifferent()
	currentTimeOffset = step.TimeOffset
	}

And I think the sorting change from #1362 and something like the if change from #2573 might also solve the original issue without having all of the confusing intermediate steps in the end result 🤔 And if we can't (which I doubt), we should at least better document the current behavior, so that the expectations of its users aren't broken.