Revamp unit tests structure

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See #202 (review):

Besides I have a few thoughts regarding tests of ckeditor4-angular in general. There is no clear distinction between unit tests and E2E tests. The only true unit tests are here. Other test files such as demo-form.component.spec.ts and simple-usage.component.spec.ts test main component indirectly and resemble E2E scenarios. On top of that we have app.e2e-spec.ts. This is quite confusing to me.