os/user: allow LookupGroupId to fail during test

On my Mac I am in group 5000 which apparently has no name
(I suspect because it is an LDAP group and I cannot reach the
LDAP server). Do not make the test fail in that case.

Fixes #14806

Change-Id: I56b11a8e86b048abfb00812eaad37802fd2adcc5
Reviewed-on: https://go-review.googlesource.com/20710Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/user/user_test.go

@@ -104,7 +104,11 @@ func TestLookupGroup(t *testing.T) {
 
 	g1, err := LookupGroupId(user.Gid)
 	if err != nil {
-		t.Fatalf("LookupGroupId(%q): %v", user.Gid, err)
+		// NOTE(rsc): Maybe the group isn't defined. That's fine.
+		// On my OS X laptop, rsc logs in with group 5000 even
+		// though there's no name for group 5000. Such is Unix.
+		t.Logf("LookupGroupId(%q): %v", user.Gid, err)
+		return
 	}
 	if g1.Gid != user.Gid {
 		t.Errorf("LookupGroupId(%q).Gid = %s; want %s", user.Gid, g1.Gid, user.Gid)