tst_qdir: Move QFINDTESTDATA() from init() to constructor.

Check result in initTestData().

Previously, QFINDTESTDATA() was repeatedly invoked in init(). The
data-driven cd() test then failed when invoked stand-alone since
cd_data(), which relies on the data path, is executed before init().

Task-number: QTBUG-40067
Change-Id: I91039247e8dcaedd92fa990f1b5f82bc54b17c60
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>