Fix warnings in tst_qquickaccessible.

Fix warnings in tst_qquickaccessible.

tst_qquickaccessible.cpp: In function 'int verifyHierarchy(QAccessibleInterface*)':
tst_qquickaccessible.cpp:70:27: warning: variable 'middleChild' set but not used
tst_qquickaccessible.cpp: In function 'QAccessibleInterface*
topLevelChildAt(QAccessibleInterface*, int, int)': tst_qquickaccessible.cpp:325:46:
warning: suggest parentheses around assignment used as truth value.

Change-Id: I80f9a1e5438279add3671828a3145af2b1f2abf8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>