Fix insignificant QScrollBar test

qtbase/tests/auto/widgets/widgets/qscrollbar/qscrollbar.pro is marked as an insignificant_test on Mac, meaning that the results are discarded and the test is not used by CI to block regressions.

Examining recent CI logs appears to show that the test failure in a stable fashion, as follows:

4/13/12 3:59:19 PM EST: Testing tst_QScrollBar
4/13/12 3:59:24 PM EST: FAIL!  : tst_QScrollBar::scrollSingleStep() Compared values are not the same
4/13/12 3:59:24 PM EST:    Actual   (testWidget->value()): 0
4/13/12 3:59:24 PM EST:    Expected (testWidget->singleStep()): 1
4/13/12 3:59:24 PM EST:    Loc: [tst_qscrollbar.cpp(102)]
4/13/12 3:59:24 PM EST: Totals: 3 passed, 1 failed, 0 skipped

Test coverage could be improved by using QEXPECT_FAIL to mark the expected failure, allowing the insignificant_test marker to be removed.