extracttest has random failures when we check properties with the libarchive
plugin. This happens because there is a race condition between
LoadJob::onFinished() (where we read some LoadJob members) and
LoadJob::onNewEntry() (where we write those members).
onFinished() is called when list() returns, in LoadJob::doWork().
But that might happen before onNewEntry() is called, since they are executed
in two different threads.
This patch puts the onFinished() call in the event queue, just like the
single-thread case does (where we emit the finished signal from
CliInterface).
An alternative could be using a DirectConnection for the entry signal, but
that is more invasive/ugly.