internal/testenv: introduce IsWindowsXP

For #23072

Change-Id: I089feafef2900413d46f2358b6e41ab78187eced
Reviewed-on: https://go-review.googlesource.com/83076Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/testenv/testenv_notwin.go

@@ -18,3 +18,7 @@ func hasSymlink() (ok bool, reason string) {
 
 	return true, ""
 }
+
+func IsWindowsXP() bool {
+	return false
+}

src/internal/testenv/testenv_windows.go

@@ -46,3 +46,12 @@ func hasSymlink() (ok bool, reason string) {
 
 	return false, ""
 }
+
+func IsWindowsXP() bool {
+	v, err := syscall.GetVersion()
+	if err != nil {
+		panic("GetVersion failed: " + err.Error())
+	}
+	major := byte(v)
+	return major < 6
+}