os: use strings.EqualFold in os_test.go

strings.EqualFold has no memory overhead and has better performance than strings.ToLower.

This is a performance test:

package bench

import (
	"strings"
	"testing"
)

func BenchmarkToLower(b *testing.B) {

	str1 := "Windows"
	str2 := "windows"

	for i := 0; i < b.N; i++ {
		if strings.ToLower(str1) == strings.ToLower(str2) {
		}
	}
}

func BenchmarkEqualFold(b *testing.B) {

	str1 := "Windows"
	str2 := "windows"

	for i := 0; i < b.N; i++ {
		if strings.EqualFold(str1, str2) {
		}
	}
}

The result:

goos: darwin
goarch: arm64
BenchmarkToLower-8      31404808                36.99 ns/op            8 B/op          1 allocs/op
BenchmarkEqualFold-8    194780793                5.989 ns/op           0 B/op          0 allocs/op
PASS

Change-Id: Id3d92534942d3eb0bdc1d01359324030ad0e434f
Reviewed-on: https://go-review.googlesource.com/c/go/+/533635
Run-TryBot: shuang cui <imcusg@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/os/os_test.go

@@ -147,7 +147,7 @@ func size(name string, t *testing.T) int64 {
 func equal(name1, name2 string) (r bool) {
 	switch runtime.GOOS {
 	case "windows":
-		r = strings.ToLower(name1) == strings.ToLower(name2)
+		r = strings.EqualFold(name1, name2)
 	default:
 		r = name1 == name2
 	}