Letting test server know not to write pwersistent data

[SindreBrurberg]

There needs to be a way to tell the testing server that it should not save persistent data as it should create error if you try to write the same data twice and so on.
The best way to fix this is not yet clear, however a simple solution is to add a Vili header that informs the server of this.

[SindreBrurberg]

in commit 4ade5f3 vili started to add a property to servers that use properties files. If the server does not contain a property it still does not differentiate on this issue.