Pop3Test: handle test failing before server thread would get created

E.g. currently on CI the pop3test fails (due to some reason unknown to me, passes locally) before the thread gets created. The test crashing on the missing thread object makes things more confusing unneededly.

See https://build.kde.org/job/Applications/job/kdepim-runtime/job/stable-kf5-qt5%20SUSEQt5.15/48/testReport/junit/projectroot.resources.pop3/autotests/akonadi_sqlite_pop3test/