CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT might be empty on the
first configuration, but because QtBuildInternalsExtra sets the
CMAKE_INSTALL_PREFIX, a second reconfiguration of a test would
stop setting the fake install prefix.
After some further consideration, there's no need to set the local
fake prefix conditionally, we can always do it (unless explicitly
opted out).
This makes sure that a reconfiguration of a test doesn't suddenly
install into the Qt prefix again.
Amends 37b132cd4e
Task-number: QTBUG-84346
Change-Id: Ic61aefe18418658455d8cdb9ebe6bcbcb8d67c99
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
2d9c9ab874