Add tests for the "onNamespaceLoaded" event

[Dumluregn]

Are you reporting a feature request or a bug?

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Provide detailed reproduction steps (if any)

In #149 we replace the logic for getNamespace() method from this repo with an external one from https://github.com/ckeditor/ckeditor4-integrations-common (they're basically the same). We had however a number of issues trying to test cases with multiple editor instances (probably because of ckeditor/ckeditor4#4390). After it's fixed it would be good to come back to this and check if we can write proper tests.