testlib: xunitxml format shows no test result if all data rows in a data-driven test are skipped

This bug is demonstrated by the testSkipSkip() test function in the qtbase/tests/auto/testlib/selftests/counting/tst_counting.cpp, where the testcase tag in the xunitxml output has a missing result attribute. The correct behaviour in this instance would be to report a pass, as there is no failure in that test function.