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 

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