test(wpmc S3): lock add_shortcode call in ShortcodeDmr::do_register

tests/ShortcodeDmrTest.php

@@ -16,4 +16,15 @@ final class ShortcodeDmrTest extends TestCase
             'ShortcodeDmr::register() must hook init'
         );
     }
+
+    public function test_do_register_registers_shortcode(): void
+    {
+        \Brain\Monkey\Functions\expect('add_shortcode')
+            ->once()
+            ->with('shortcode_dmr', [ShortcodeDmr::class, 'render']);
+
+        ShortcodeDmr::do_register();
+
+        self::assertTrue(true);
+    }
 }