Add excludes for log4j.xml to test jar generation

While working on the ISPN cache provider, I've realised that the test
jar contains a log4j.xml and this stops you from using a different log4j
settings to the one in the test jar. This is because log4j first and
foremost looks for a 'log4j.xml' and if present, it uses that. This
cannot be overriden with -Dlog4j.configuration.

As a result, I'm adding an excludes to avoid this log4j.xml ending up in
the test jar. The other alternative would be to rename
src/test/resources/log4j.xml to something else, i.e.
log4j-infinispan.xml but doing this would stop the log4j settings from
being picked up by default during Infinispan development, hence, I think
the excludes setting is the best option here.

If anyone has any issues with this, let me know.