Add QUnit tests for the parsing of custom namespace routes. Add a custom schema fixture based on the `wp-js-widgets` plugin. Test that the client can parse the widget namespace in the schema and correctly construct the expected group of models and collections. Also includes a small unrelated QUnit fix to ensure nav-menu test passes when it is loaded without its tests executing as well as a small jshint fix, adding a missing semicolon since [40107].
Props jnylen0.
Fixes#39561.
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