Get rid of ~1000 useless log files out of 10.000 tests in /tmp.
* bzrlib/tests/test_selftest.py: (TestRunner.test_skipped_log_deleted, TestRunner.test_not_aplicable_log_deleted, TestRunner.test_known_failure_log_deleted): Add tests checking that the log is deleted for skipped, not applicable and known failures tests.
* bzrlib/tests/__init__.py: (ExtendedTestResult.addError, ExtendedTestResult.addFailure, ExtendedTestResult.addSuccess): Keep log only on true failures and errors.