Follow-Ups of e2e-testing

[kulla]

This Issue contains a list of todos I noticed during implementing https://github.com/serlo/serlo.org/issues/222:

• [ ] Some h2 headings are of type <div class="h2"> instead of <h2>: https://github.com/serlo/serlo.org/blob/5d809f5fe712c3cb4e24e52dccd6d67297775054/packages/public/server/__tests-e2e__/taxonomy.ts#L67
• [ ] Some buttons are only visible by class definitions, e.g. https://github.com/serlo/serlo.org/blob/5d809f5fe712c3cb4e24e52dccd6d67297775054/packages/public/server/__tests-e2e__/taxonomy.ts#L107
• [ ] There need to be a better way to address elements in the new editor: https://github.com/serlo/serlo.org/blob/5d809f5fe712c3cb4e24e52dccd6d67297775054/packages/public/server/__tests-e2e__/taxonomy.ts#L122
• [ ] Puppeteer should run in incognito browser context when run in headless mode with more than one worker
• [x] Better logout function: https://github.com/serlo/serlo.org/blob/79363424aa2160539c1677c5a7b8f5e06ad59563/packages/public/server/__tests-e2e__/entities/video.ts#L76 (shall only logout when logged in -> cookie authenticated is set)
• [ ] Omit double clicking in new editor: https://github.com/serlo/serlo.org/blob/79363424aa2160539c1677c5a7b8f5e06ad59563/packages/public/server/__tests-e2e__/entities/video.ts#L106-L107
• [ ] Use role here: https://github.com/serlo/serlo.org/blob/79363424aa2160539c1677c5a7b8f5e06ad59563/packages/public/server/__tests-e2e__/entities/video.ts#L114-L115

[kulla]

• [ ] Developer tools in bottom position