net: make getmac based tests on windows more robust

The Windows 7 getmac command may report the physical address of an adapter
as "Disabled" or "N/A". Handle these two cases to make the tests more
robust when building on Windows with manually disabled adapters or turned
off hardware.

Addresses issue #14130.

Change-Id: I0c2f8554b4b6810568e4e60ed53857599401f296
Reviewed-on: https://go-review.googlesource.com/19411Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/net_windows_test.go

@@ -487,8 +487,13 @@ func TestInterfaceHardwareAddrWithGetmac(t *testing.T) {
 	//
 	//Connection Name:  Bluetooth Network Connection
 	//Network Adapter:  Bluetooth Device (Personal Area Network)
-	//Physical Address: XX-XX-XX-XX-XX-XX
-	//Transport Name:   Media disconnected
+	//Physical Address: N/A
+	//Transport Name:   Hardware not present
+	//
+	//Connection Name:  VMware Network Adapter VMnet8
+	//Network Adapter:  VMware Virtual Ethernet Adapter for VMnet8
+	//Physical Address: Disabled
+	//Transport Name:   Disconnected
 	//
 	want := make(map[string]string)
 	var name string
@@ -516,6 +521,9 @@ func TestInterfaceHardwareAddrWithGetmac(t *testing.T) {
 			if addr == "" {
 				t.Fatal("empty address on \"Physical Address\" line: %q", line)
 			}
+			if addr == "disabled" || addr == "n/a" {
+				continue
+			}
 			addr = strings.Replace(addr, "-", ":", -1)
 			want[name] = addr
 			name = ""