Testlib: Replace linked lists in QTestTablePrivate by std::vector.

This speeds up tests/auto/corelib/tools/qchar which spends a lot
of time in QTestTablePrivate::elementAt(), QTestTablePrivate::dataAt()
from 6s to 500ms on an average Linux machine.

Task-number: QTBUG-38890
Change-Id: I16b75819f88ca7974a581affd8409f7b4038a712
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>