test: make maplinear iterdelete test less flaky

iterdelete's run time varies; occasionally we get unlucky. To reduce spurious failures, average away some of the variation.

On my machine, 8 of 5000 runs (0.15%) failed before this CL. After this CL, there were no failures after 35,000 runs.

I confirmed that this adjusted test still fails before CL 141270043.

LGTM=khr
R=khr
CC=bradfitz, golang-codereviews
https://golang.org/cl/140610043

test/maplinear.go

@@ -146,7 +146,10 @@ func main() {
 	// O(n lg n) time.  Fortunately, the checkLinear test
 	// leaves enough wiggle room to include n lg n time
 	// (it actually tests for O(n^log_2(3)).
-	checkLinear("iterdelete", 10000, func(n int) {
+	// To prevent false positives, average away variation
+	// by doing multiple rounds within a single run.
+	checkLinear("iterdelete", 2500, func(n int) {
+		for round := 0; round < 4; round++ {
 			m := map[int]int{}
 			for i := 0; i < n; i++ {
 				m[i] = i
@@ -157,5 +160,6 @@ func main() {
 					break
 				}
 			}
+		}
 	})
 }