os: skip TestHostname on Plan 9

TestHostname was re-enabled in CL 10753.
However, on Plan 9 the hostname is not obtained
by executing a "hostname" command, but by reading
the #c/sysname file.

Change-Id: I80c0e303f4983fe39ceb300ad64e2c4a8392b695
Reviewed-on: https://go-review.googlesource.com/11033
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -1251,7 +1251,10 @@ func testWindowsHostname(t *testing.T) {
 func TestHostname(t *testing.T) {
 	// There is no other way to fetch hostname on windows, but via winapi.
 	// On Plan 9 it can be taken from #c/sysname as Hostname() does.
-	if runtime.GOOS == "windows" {
+	switch runtime.GOOS {
+	case "plan9":
+		t.Skipf("skipping on %s", runtime.GOOS)
+	case "windows":
 		testWindowsHostname(t)
 		return
 	}