Issues if a test exits before xml is created

[baraserg]

If for any reason a google test exits with exit(1) or before the test ends, code will still try to read the xml file, but since it doesn't exists a FileNotFound exception is raised.

https://github.com/pytest-dev/pytest-cpp/blob/master/src/pytest_cpp/google.py#L89
https://github.com/pytest-dev/pytest-cpp/blob/master/src/pytest_cpp/google.py#L106

[baraserg]

@dajose