Usually server test go through different phases.
Private: This is only intended for tibia staff, senators, tutors, fansite representatives and in this case, players with a lvl +400 in their account.
Public: A few days after the private test is released, it gets open to all public, usually only premium accounts but sometimes free accounts are also allowed during the last days of the server test. Most of the times I remember, the server test starts as Optional pvp, then Open PvP and by the last days it could also be hardcore PvP.
Last day: During the last day, usually cipsoft staff spawns plenty of bosses such as orshabaal, ferumbras and apocalypse in the streets of Thais so players can have fun trying to kill them.
For me, server test are a great way to try out new hunting areas without any risk.