Add test around XMLTestRunLogger filename logging

Since the aim of this branch is to modify the behavior of the logging
target of the XMLTestRunLogger, there should be a test confirming the
original intended behavior. Which in this case is logging the results to
a filename provided upon instantiation.
Make XMLTestRunlogger accept file identifiers

Every test run monitor accepts file identifiers as targets instead of
filenames. The `XMLTestRunLogger` now accepts both, to make sure that it
can be easily substituted for other test run monitors
Merge pull request #11 from bauglir/xmltestrunlogger-api-compatibility

Improve XMLTestRunLogger API compatibility