Run karma tests with local `ckeditor-dev` package

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The karma.conf.js file now uses CDN to load ckeditor package for tests. It makes sense in many cases because the integration will use already released packages, however it makes it unable to test integration with some local changes in ckeditor-dev package.

We should enable testing with local ckeditor-dev package.

Related to https://github.com/ckeditor/ckeditor4-angular/issues/8.

[f1ames]

See further discussion in https://github.com/ckeditor/ckeditor4-angular/issues/8 starting from https://github.com/ckeditor/ckeditor4-angular/issues/8#issuecomment-491217267.