test: add test case for PrivateDevices=y and Group=daemon

For root, group enforcement needs to come after PrivateDevices=y set up
according to 096424d123. Add a test to
verify this is the case.

test/meson.build

@@ -102,6 +102,7 @@ test_data_files = '''
         test-execute/exec-privatedevices-no-capability-mknod.service
         test-execute/exec-privatedevices-no-capability-sys-rawio.service
         test-execute/exec-privatedevices-no.service
+        test-execute/exec-privatedevices-yes-with-group.service
         test-execute/exec-privatedevices-yes-capability-mknod.service
         test-execute/exec-privatedevices-yes-capability-sys-rawio.service
         test-execute/exec-privatedevices-yes.service

test/test-execute/exec-privatedevices-yes-with-group.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Test Group=group is applied after PrivateDevices=yes
+
+[Service]
+PrivateDevices=yes
+Group=daemon
+Type=oneshot
+
+# Check the group applied
+ExecStart=/bin/sh -x -c 'test "$$(id -n -g)" = "daemon"'
+
+# Check that the namespace applied
+ExecStart=/bin/sh -c 'test ! -c /dev/kmsg'
+
+# Check that the owning group of a node is not daemon (should be the host root)
+ExecStart=/bin/sh -x -c 'test ! "$$(stat -c %%G /dev/stderr)" = "daemon"'