os/user: simplify test skip for plan9

There's no need to specifically check for runtime.GOOS as there's
already a generic mechanism for that.

Change-Id: I7125443ead456548bd503c5e71cd56e9eb30b446
Reviewed-on: https://go-review.googlesource.com/c/go/+/330750
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>

Author: kolyshkin

src/os/user/lookup_plan9.go

@@ -18,6 +18,7 @@ const (
 )
 
 func init() {
+	userImplemented = false
 	groupImplemented = false
 }
 

src/os/user/user_test.go

@@ -56,10 +56,6 @@ func compare(t *testing.T, want, got *User) {
 func TestLookup(t *testing.T) {
 	checkUser(t)
 
-	if runtime.GOOS == "plan9" {
-		t.Skipf("Lookup not implemented on %q", runtime.GOOS)
-	}
-
 	want, err := Current()
 	if err != nil {
 		t.Fatalf("Current: %v", err)
@@ -77,10 +73,6 @@ func TestLookup(t *testing.T) {
 func TestLookupId(t *testing.T) {
 	checkUser(t)
 
-	if runtime.GOOS == "plan9" {
-		t.Skipf("LookupId not implemented on %q", runtime.GOOS)
-	}
-
 	want, err := Current()
 	if err != nil {
 		t.Fatalf("Current: %v", err)