Revert "tests: fixed compilation of tst_qtextstream"

This reverts commit 9a6a58a9

,
which was a work-around for a qmake bug, now fixed.

Task-number: QTBUG-19393
Change-Id: Id467bb5907a88f03eac0e29a90f4ff7e97045423
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>