[Codeowners] Add missing test ownership

.github/CODEOWNERS

@@ -121,6 +121,7 @@
 /modules/mbedtls/tests/                       @godotengine/network @godotengine/tests
 /modules/multiplayer/                         @godotengine/network
 /modules/multiplayer/doc_classes/             @godotengine/network @godotengine/documentation
+/modules/multiplayer/tests/                   @godotengine/network @godotengine/tests
 /modules/upnp/                                @godotengine/network
 /modules/upnp/doc_classes/                    @godotengine/network @godotengine/documentation
 /modules/webrtc/                              @godotengine/network