test: add end-to-end tests, fix #2249

[flaurida]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

Resolves #2249. Adds initial end-to-end tests of VuePress. Specifically, tests that a user can 1) create a new project, 2) view the project in the browser, 3) add a page to it, and 4) see the updated project in the browser.

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

The end-to-end tests run in Chromium, Firefox, and WebKit as these are the browsers Playwright supports.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

This replaces #2258 as I updated my master branch. Added end-to-end tests to CircleCI.

I saved the tests in a new __e2e__ directory. I am happy to move them elsewhere if you like.

Thanks for reviewing, looking forward to your feedback! 😊