Re-add "text drag&drop still works" regression test.

[mrdrogdrog]

Which part of the project should be enhanced? Drag'n'Drop Tests

Is your enhancement request related to a problem? Please describe. While implementing hedgedoc/react-client#1787 I removed the "Drag&Drop of plain text in the editor still works" regression test. This test was added while implementing the "drag and drop file to upload" feature because for a short time the feature broke the dnd of plain text. I really wanted to keep it and tried a lot of things but it just didn't work.The test either hasn't worked at all or weird side effects like duplicated text appeared. At some point I just gave up and removed the test. Maybe it's a bug in cypress, maybe in codemirror. I have no idea.

Describe the solution you'd like Reimplement the test.

Describe alternatives you've considered Don't reimplement the test. CodeMirror 6 handles input different from CodeMirror 5 so maybe this test isn't needed anymore.