Avoid using cross symbol to denote "success but with known issues"

If a test suite passes but has known issues currently swift-testing uses either `xmark.diamond` on macOS or otherwise `HEAVY BALLOT X` to denote the status. This is confusing since it leads to log output that looks contradictory: ``` ✘ Test run with 5 tests in 2 suites passed after 3.235 seconds with 1 known issue ``` IMO the fact that there are known issues is useful additional information, but the overall status is still success so it should just be a tick, same as if there were no known issues (or maybe there could be an alternative tick icon if we really care about the distinction)