[autotests] tst_qcombobox fails too often in CI

The tst_qcombobox autotest has failed 9 times in CI between 2013-03-25 and 2013-04-8, and ranks as one of the top most failing test in that time frame. It seems likely that the test needs fixing.

Most common failures:

• tst_QComboBox::task190205_setModelAdjustToContents() Compared values are not the same
• tst_QComboBox::keyBoardNavigationWithMouse() 'selection > 3' returned FALSE. ()

See more info in the following test results:

• http://testresults.qt-project.org/ci/QtBase_dev_Integration/build_00717/win64-msvc2012_developer-build_qtnamespace_Windows_8/log.txt.gz
• http://testresults.qt-project.org/ci/QtBase_stable_Integration/build_00747/win64-msvc2012_developer-build_qtnamespace_Windows_8/log.txt.gz
• http://testresults.qt-project.org/ci/QtBase_stable_Integration/build_00745/win64-msvc2012_developer-build_qtnamespace_Windows_8/log.txt.gz
• http://testresults.qt-project.org/ci/QtBase_dev_Integration/build_00771/win32-mingw47_developer-build_qtlibinfix_Windows_7/log.txt.gz
• http://testresults.qt-project.org/ci/QtBase_dev_Integration/build_00751/win64-msvc2012_developer-build_qtnamespace_Windows_8/log.txt.gz