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.

[come-nc]

@julien-nc is stable28 testing still needed?
tests/autoload.php was backported until stable29 but no further.
If really needed I can backport it to stable28 but as it’s EOL I think dropping the test is easier?

[julien-nc]

@come-nc Yep, stable28 testing can be dropped.