Each test function is responsible for removing the children it added to
m_window, since m_window is reused for each testcase.
Due to this, the test
QCOMPARE(children.length(), 2);
failed, (it was 3).
Change-Id: Idbd13214d2e841450800df0542fbdbcfaac85ce1
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>