Multiple system outs in XML output

[daniel-v]

According to JUnit v4 XSD, having multiple system-out for a test-case is invalid.

Example to invalid XML:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite errors="0" failures="0" tests="6" skipped="0" name="test.ipsums" timestamp="2017-02-21T15:02:29">
    <properties>
      <property name="platform" value="dartium" />
    </properties>
    <testcase classname="ipsum" name="habla hable" time="0.015" />
    <system-out>Muhaha</system-out>
    <system-out>Another line</system-out>
  </testsuite>
</testsuites>

The converter should merge the prints of that belong to the same testsuite.