Improve feature parity between NUnit and JUnit logger: system-out and system-err

[siegfriedpammer-qt]

Is your feature request related to a problem? Please describe.
JunitXmlSerializer provides both stdout and stderr in the XML.

https://github.com/spekt/testlogger/blob/v6.1.0/src/JUnit.Xml.TestLogger/JunitXmlSerializer.cs#L276-L306

NUnitXmlSerializer does not.

https://github.com/spekt/testlogger/blob/v6.1.0/src/NUnit.Xml.TestLogger/NUnitXmlSerializer.cs#L259-L271

Describe the solution you'd like
Improve feature parity by copying those lines from JunitXmlSerializer to NUnitXmlSerializer

Describe alternatives you've considered
I am able to use --logger:"junit;LogFilePath=path/to/logfile.xml" but it feels somewhat weird to use a "JUnit" library with an "NUnit" project.

Additional context
I am reporting this because I was surprised that it didn't work with the NUnit variant and I want to make sure that future readers/users of this library can at least find this issue documenting this discrepancy.

[djeikyb]

not a contributor, but these are two different report formats, one specified by the junit project, the other specified by the nunit project. if you want the nunit report specification to change, talk to the nunit folks