runtime: add timing test for iterate/delete map idiom.

LGTM=bradfitz, iant
R=iant, bradfitz
CC=golang-codereviews
https://golang.org/cl/140510043

test/maplinear.go

@@ -140,4 +140,22 @@ func main() {
 			m[complex(float64(i), float64(i))] = 1
 		}
 	})
+
+	// ~70ms on a 1.6GHz Zeon.
+	// The iterate/delete idiom currently takes expected
+	// 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) {
+		m := map[int]int{}
+		for i := 0; i < n; i++ {
+			m[i] = i
+		}
+		for i := 0; i < n; i++ {
+			for k := range m {
+				delete(m, k)
+				break
+			}
+		}
+	})
 }