core: Use a subdirectory of /run/ for PrivateDevices=

When we're starting early boot services such as systemd-userdbd.service,
/tmp might not yet be mounted, so let's use a directory in /run instead
which is guaranteed to be available.

src/test/test-namespace.c

@@ -206,6 +206,7 @@ TEST(protect_kernel_logs) {
                                     NULL,
                                     NULL,
                                     NULL,
+                                    RUNTIME_SCOPE_SYSTEM,
                                     NULL);
                 assert_se(r == 0);
 

src/test/test-ns.c

@@ -108,6 +108,7 @@ int main(int argc, char *argv[]) {
                             NULL,
                             NULL,
                             NULL,
+                            RUNTIME_SCOPE_SYSTEM,
                             NULL);
         if (r < 0) {
                 log_error_errno(r, "Failed to set up namespace: %m");