Are cosmetics which are tested on animals considered vegan even if they only contain plant-based ingredients?
In my understanding they are not considered to be vegan.
Vegans, in general, try to avoid the exploitation of animals for human ends.
Testing of animals is considered to be exploitation and harmful to animals. It's not in the letter or spirit of veganism, as it's defined for example by The Vegan Society, to buy or use those products, and if they are labelled "vegan" then you should complain to the manufacturer.
See for example The Vegan Society's shopping advice page where they explain what their trademark shows:
Vegans also avoid using animals for shoes, clothing, household products, toiletries and cosmetics. Be sure to read labels carefully, or look for our Vegan Trademark. This internationally-recognised symbol shows that a product does not contain animal products and has not been tested on animals.