#30 fix XML reporter output and add more JUnit XML elements

[Dirk Jagdmann]

this patch fixes the XML output of the XML reporter. The old version used copies of the FILE pointer and resulted in the child and parent processes or the running test suite to clobber each other's output. I've fixed that by opening the XML file and appending whenever something should be written to it.

I also made use of the reported runtime of a test. Since the runtime is only available after a test has been run, but in XML it needs to appear in the attributes of the opening element, I implemented a stack of file positions and outputting whitespace to leave room for those attributes. When the test/suite is done the output file is seeked to the correct position and the attributes are written.

This patch needs the changes from [#29].