chore(tests): Cleanup bootstrap.php to be forward-compatible

[come-nc]

This uses the new tests/autoload.php from nextcloud/server instead of messing directly with OC private autoloader.

See nextcloud/server#52951 and nextcloud/server#52945 for context.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.58%. Comparing base (699f8f7) to head (04d1191).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2753   +/-   ##
=========================================
  Coverage     45.58%   45.58%           
  Complexity      984      984           
=========================================
  Files            79       79           
  Lines          3565     3565           
=========================================
  Hits           1625     1625           
  Misses         1940     1940           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.