Fix failing unit tests

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In #441, I commented out some unit tests to get the pipeline to work for now. However, this should of course only be of a temporary nature. These tests should be fixed in the future:

• spec/models/medium_publisher_spec.rb: test with title "returns an assignment title error if assignment is to be created but title of an already existing assignment in the lecture is given"

• spec/requests/media_spec.rb: commented out tests starting with describe "#search_by" do

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Not relevant anymore as #441 does not disable these tests anymore (as originally done in the PR).