Unfortunately, we can't simply use session_id() to regenerate the
session id in unit tests. Starting from PHP 7.2, it would trigger
"session_id(): Cannot change session id when headers already sent",
refer to MDL-60978 and PHP bug #75628 for more details.
As a workaround, we use a static property allowing us to inject the
value that we then use as a session identifier. This is reasonably
enough to make sure that the identifier is used as a part of the key
prefix.
David Mudrák
9fb92df867