Allow the test to modify the suite name

the functions oneTimeTearDown and oneTimeSetUp must not be piped. That
is important in order to allow the called code to modify variables such
as the suite name.
Bug fix to avoid appending to existing file

There is no reason to append here. If the file already exist appending
will result in a broken file.
Give possibility to have details about failures

In JUnit xml format, one can have the value of a tag failure populated
with the details, such as the stack trace.
Merge pull request #1 from Barthelemy/master

3 changes for the xml-junit branch