os/user: enable tests on all supported platforms

All of the currently supported platforms have a working user
implementation and do not use stubs. As a result, enable the tests
on all platforms rather than whitelisting.

R=golang-dev, dave, iant
CC=golang-dev
https://golang.org/cl/14454044

src/pkg/os/user/user_test.go

@@ -13,12 +13,6 @@ func check(t *testing.T) {
 	if !implemented {
 		t.Skip("user: not implemented; skipping tests")
 	}
-	switch runtime.GOOS {
-	case "linux", "freebsd", "darwin", "windows", "plan9":
-		// test supported
-	default:
-		t.Skipf("user: Lookup not implemented on %q; skipping test", runtime.GOOS)
-	}
 }
 
 func TestCurrent(t *testing.T) {