Provide disposable, pre-provisioned XWiki instances for integration tests

Xinit has no automated tests, and testing it by hand needs a real XWiki installation for each supported setup.

Provide a script that builds and runs containers with systemd as PID 1 and one official XWiki Debian package (xwiki-tomcat10-pgsql, xwiki-xjetty-pgsql…), already past the Distribution Wizard (flavor installed, Admin and standard users), plus helpers that inject faults (frozen or killed JVM, database down, heap and kernel OOM, slow pages…).