chore(cli/test_admin_ui): use a real built dashboard

[SkyeYoung]

Description

Originally, the test of dashboard in cli used simulated files, which may not detect the real built dashboard's problems.

This PR is improved to use the real built dashboard directly to ensure the relevance of the issue and the dashboard.

At the same time, the check for static resource files svg has been added to better cover the needs of the dashboard.

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Copilot]

Pull Request Overview

This PR updates the admin UI test to use a real built dashboard instead of simulated test files. The change ensures better validation of the actual dashboard functionality by cloning and building the apisix-dashboard repository, then testing against real static assets instead of mock files.

Key changes:

• Added build process to clone and compile the actual apisix-dashboard during tests
• Enhanced test coverage to include SVG file accessibility checks
• Replaced mock test files with real dashboard dist files

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[juzhiyuan]

LGTM