fix: intermittent failures in playground/server-origin test

[tony19]

fix #9849

Description

Updates playground/vue-server-origin test to use localhost instead of https://vue-server-origin.test, which causes intermittent failures as the test browser tries to resolve it.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[tony19]

The Mixed content and Redirect rules checks are stuck for some reason. They've been running since 2d ago.

[bluwy]

Looks like an issue on Netlify end. Thanks for the fix!