scrape: fix two loop variable scoping bugs in test

[rsc]

Consider code like:

for i := 0; i < numTargets; i++ {
	stopFuncs = append(stopFuncs, func() {
		time.Sleep(i*20*time.Millisecond)
	})
}

Because the loop variable i is shared by all closures, all the stopFuncs sleep for numTargets*20 ms.

If the i were made per-iteration, as we are considering for a future Go release, the stopFuncs would have sleep durations ranging from 0 to (numTargets-1)*20 ms.

Two tests had code like this and were checking that the aggregate sleep was at least numTargets*20 ms
("at least as long as the last target slept"). This is only true today because i == numTarget during all the sleeps.

To keep the code working even if the semantics of this loop change, this PR computes

d := time.Duration((i+1)*20) * time.Millisecond

outside the closure (but inside the loop body), and then each closure has its own d. Now the sleeps range from 20 ms to numTargets*20 ms, keeping the test passing
(and probably behaving closer to the intent of the test author).

The failure being fixed can be reproduced by using the current Go development branch with

GOEXPERIMENT=loopvar go test

[roidelapluie]

Thank you for providing such a comprehensive explanation and the solution!