syscall: fix TestGroupCleanupUserNamespace on CentOS

Update the list of expected "id" outputs in
TestGroupCleanupUserNamespace with SELinux context information as used
on CentOS.

Fixes #34547

Change-Id: I426bbe2d04e2039c87490362a1891ec3de6e36e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/197841
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/exec_linux_test.go

@@ -309,6 +309,7 @@ func TestGroupCleanupUserNamespace(t *testing.T) {
 		"uid=0(root) gid=0(root) groups=0(root),65534(nogroup)",
 		"uid=0(root) gid=0(root) groups=0(root),65534",
 		"uid=0(root) gid=0(root) groups=0(root),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody)", // Alpine; see https://golang.org/issue/19938
+		"uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023",                                                                               // CentOS with SELinux context, see https://golang.org/issue/34547
 	}
 	for _, e := range expected {
 		if strOut == e {