Set up `XMLRunLoggerTest` environment correctly

Although the proper parent class was used to execute the
`XMLRunLoggerTest` in the context of the `helper_classes` directory, the
`setUp` and `tearDown` methods of that class were shadowed by those
defined on `XMLRunLoggerTest`. This caused the tests to fail, since the
test suite could not be found.
Merge pull request #12 from bauglir/fix-xmltestrunlogger-test-environment

Set up XMLRunLoggerTest environment correctly