Output with multiple test loggers from a single test run

With the new (4.6) testlogger, all test events are buffered until the test ends, at which point the log is generated and output. Therefore it should be reasonably simple to generate the log multiple times using different testloggers.

This would allow us to, for example, generate xunit XML for use with simplistic result parsing tools like Pulse, while at the same time generating logs with the more expressive native testlib format for more detailed results parsing.

Running a test might look something like: ./tst_qstring -xml:testresults/tst_qstring.xml -xunitxml:testresults/tst_qstring_xunit.xml