xUnit test result parser fails to add more than one <testsuite> element

When trying to add xUnit test results with more than one <testsuite> element, Sonar fails because of a duplicated key.

Root cause message from Waleri Enns:
Had a look at it and realized that the issue is somewhat more
complicated. The whole story: we are using a somewhat modified copy of
the surefire-plugin (for historical reasons). It saves the measures in
context of resources which are found using the value of the classname
attribute. It works well for Java & Junit becase the classname attribute
contains the "full qualified" "dotted" classname which can be used to
find a resource.

Not in our case, though: classname contains just the name of the
testclass, we're trying to find the resource and get "null". It serves
us well the first time but further measures with "null" key are
considered as a duplicate by sonar core.

Furthermore, the surefire-reports and xunit-reports in "SampleProject"
seem to contain only one testsuite-tag. Which explains why the problem
doesnt arise there.