I'd make use of a FileHandler which would roll the file when a system property changes.

For use case, see ARQ-710.

The handler would have a property called, say, fileNameFormat, which would be a string like:

my-server-log-${arq.currentTest}-${arq.containerId}.txt

The handler would replace the string every time and compare with previous; when different, it would roll the file.

It would not be super efficient but good enough for the purposes of testing.
(It would not be the default handler, only configured on demand.)