feat(tests): add e2e tests for widgets

[pixeldesu]

What

This PR adds 22 end-to-end test cases for adding all widgets to the widget sidebar.

3 basic tests:

• checking if the Edit widgets button is present
• checking if the select is visible after Edit widgets has been pressed (Edit mode)
• checking if removing widgets works

19 tests for all current widgets

Why

After the recent merge of the widget refactoring/lint issues a lot of regressions happened. Testing if widgets are present after being added at least makes sure that they are not breaking the application completely.

Additional info (optional)

• I added mk-widget-edit/select/add classes to mark up the elements that are used to add widgets
• I added more mkw-* classes to the widgets that had them missing
• The test cases do not check for specific widget functionality, they only check that the wrapped mkw-{widgetName} is present after being added

[syuilo]

Cool!
Do you know why the e2e test is failing?

[pixeldesu]

@syuilo No clue 😕

Before the final push, that case worked...so I assume that it's something timeout-related mainly and we might need to adjust something here so that it consistently works.

Here's the last action run on my fork before I made the PR, and there it passed fine!
https://github.com/pixeldesu/misskey/runs/6603509947?check_suite_focus=true

This also might be an issue with the basic testsuite, as the beforeEach-hook is the same there.

[syuilo]

👍🏻👍🏻👍🏻