Add failing test only when XML is complete

Otherwise we can get this error:

WARNING:root:Test suite failed with: Command /home/tomeu/virgl-prefix/source/VK-GL-CTS/build/modules/gles3/deqp-gles3 --deqp-stdin-caselist --deqp-surface-type=window --deqp-gl-config-name=rgba8888d24s8ms0 --deqp-log-images=disable --deqp-visibility=hidden --deqp-watchdog=enable --deqp-surface-width=256 --deqp-surface-height=256 --deqp-log-filename=/tmp/gles3-master.txt-logs/None_hasty_17000.log returned non-zero exit status WARNING:root:Results may be incomplete Traceback (most recent call last): File "./deqp_runner.py", line 587, in runner.run_once(opts) File "./deqp_runner.py", line 521, in run_once test_results, failing_test = self._run_once(test_cases) File "./deqp_runner.py", line 462, in _run_once test_results = self._run_tests_hasty(test_cases, failing_test) File "./deqp_runner.py", line 447, in _run_tests_hasty detailed_results=detailed_results) File "./deqp_runner.py", line 112, in _parse_test_results failing_test.append(test_case) UnboundLocalError: local variable 'test_case' referenced before assignment