net: skip Windows test using getmac if getmac cmdlet not available

This doesn't appear to be present on Windows Server 2012 or 2016:

https://build.golang.org/log/6ea21b99c9b8a2be20f9aeaec6c425b84faf1af7
https://build.golang.org/log/2bcf04f1df003577352f4f987a39a59a081094ee

Updates golang/go#17513
Updates golang/go#20073

Change-Id: I72820704b4cb16bb1720b7f6a9f2e10028c71334
Reviewed-on: https://go-review.googlesource.com/41395Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/net/net_windows_test.go

@@ -503,21 +503,26 @@ func TestInterfaceAddrsWithNetsh(t *testing.T) {
 	}
 }
 
-func getmacSpeaksEnglish(t *testing.T) bool {
+// check that getmac exists as a powershell command, and that it
+// speaks English.
+func checkGetmac(t *testing.T) {
 	out, err := runCmd("getmac", "/?")
 	if err != nil {
+		if strings.Contains(err.Error(), "term 'getmac' is not recognized as the name of a cmdlet") {
+			t.Skipf("getmac not available")
+		}
 		t.Fatal(err)
 	}
-	return bytes.Contains(out, []byte("network adapters on a system"))
+	if !bytes.Contains(out, []byte("network adapters on a system")) {
+		t.Skipf("skipping test on non-English system")
+	}
 }
 
 func TestInterfaceHardwareAddrWithGetmac(t *testing.T) {
 	if isWindowsXP(t) {
 		t.Skip("Windows XP does not have powershell command")
 	}
-	if !getmacSpeaksEnglish(t) {
-		t.Skip("English version of getmac required for this test")
-	}
+	checkGetmac(t)
 
 	ift, err := Interfaces()
 	if err != nil {