selinux: check mac_selinux_init() in tests

Since bc8d57f290 mac_selinux_init() is checked and considered fatal
tree-wide.

Coverity complains about it not being checked in the test code.

Follow-up of: #16223
Fixes: CID 1429975

src/test/test-udev.c

@@ -82,7 +82,10 @@ static int run(int argc, char *argv[]) {
         }
 
         log_debug("version %s", GIT_VERSION);
-        mac_selinux_init();
+
+        r = mac_selinux_init();
+        if (r < 0)
+                return r;
 
         action = argv[1];
         devpath = argv[2];