All the setup/teardown/pre/post/conditions template methods
now are required to return void. This was warned with phpunit 7
and now is enforced.
At the same time, fix a few wrong function names,
provider data and param types, return statements...
Based on repository_owncloud from
https://github.com/learnweb/moodle-repository_owncloud at 5b5fdbb.
Differences to the original:
* Renamed to repository_nextcloud
* Updated version.php for core
* Removed compatibility polyfills (webdav, privacy)
* Removed boilerplate files (e.g. README, CI config)
* Implement null_provider for privacy API as personal data is neither
stored nor transmitted to the external system
