runtime: simplify range expressions in tests

Generated by running gofmt -s on the files in question.

Change-Id: If6578b150e1bfced8657196d2af01f5d36879f93
Reviewed-on: https://go-review.googlesource.com/100135
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/rand_test.go

@@ -25,7 +25,7 @@ func BenchmarkFastrandHashiter(b *testing.B) {
 	}
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {
-			for _ = range m {
+			for range m {
 				break
 			}
 		}

src/runtime/string_test.go

@@ -125,7 +125,7 @@ func BenchmarkRuneIterate(b *testing.B) {
 		for _, sd := range stringdata {
 			b.Run(sd.name, func(b *testing.B) {
 				for i := 0; i < b.N; i++ {
-					for _ = range sd.data {
+					for range sd.data {
 					}
 				}
 			})
@@ -135,7 +135,7 @@ func BenchmarkRuneIterate(b *testing.B) {
 		for _, sd := range stringdata {
 			b.Run(sd.name, func(b *testing.B) {
 				for i := 0; i < b.N; i++ {
-					for _, _ = range sd.data {
+					for range sd.data {
 					}
 				}
 			})